Boiling and TwoPhase Flow of Mixtures

Chemical Engineering

Boiling and Two-Phase Flow of Mixtures

Abstract

The purpose of this report is to investigate the boiling and two-phase flow of mixtures, particularly for use in cooling and refrigeration applications. A great deal of information is currently available for pure component interactions, however, less information is presented for mixtures. After consulting and reviewing the literature it was found that many mixture combinations exist capable of replacing pure components. Advantages exist in tailoring the mixture to the application in order to increase safety and overall efficiency and reduce undesirable characteristics. By choosing different properties of the mixture components: greater heat fluxes can be dissipated, thereby increasing the efficiency of the system and reducing requirements such as energy, working fluid volume and surface area. Disadvantages exist in the use of zeotropic mixtures. If zeotropic mixtures are not designed for, then poor operation will occur. This is due to undesirable flow conditions affecting the overall heat transfer of the system, and separation of the mixture into a liquid and vapour phase. Composition variations within the working fluid can also cause unexpected operation and must be accounted for.

It is seen that some mixtures can be used as "drop-in" working fluids for existing applications, however for best results it is more advantageous to design the system with the working fluid in mind from the beginning. This allows maximum efficiency and economic potentials to be met.

1.0 Introduction

The term cooling refers to any process in which thermal energy is transferred via radiation, conduction or convection. There are many processes and applications of this in everyday life, from computing to chemical processes. There is a great understanding of the present technology of heat transfer and of the working fluids involved. However there are new regions of interest such as micro applications in computers and applications that require "super heat fluxes" to be dissipated. Additional interest is in further improvements that could be made to existing processes by altering the operating conditions. Possibly the easiest and most effective change that could be made is to the operating fluid, differing the fluid changes the properties of the cooling unit, these changes can then be employed to yield more effective results. Changing the fluid may be done for several reasons; to increase efficiency, decrease operating costs, reduce environmental impact and increase safety.Many techniques are currently available to transfer heat, these include: air conditioning, air cooling, cryogenics, conduction, solar cells, liquid cooling, refrigeration and thermoelectric cooling. These techniques take place in many different devices, such as: heat exchangers, radiators, evaporators, intercoolers, cooling towers and steam generators. The vast majority of heat exchange operations use liquids in order to transfer this heat. This is due to the ease of transporting a liquid through the system compared to a solid, and the heat transfer coefficient being significantly higher than that for a gas. The properties of these working fluids are important in overall efficiency of the process. Many applications use pure components in the cooling system, however the possibilities and range of physical properties are expanded by using a mixture of components. Using different fluids will yield different results and efficiencies. It is important to investigate the advantages and disadvantages of each option. This report aims to investigate the use of mixtures in refrigeration and cooling processes, then assess the advantages and disadvantages of utilising mixtures in such a role.

2.0 Use of Liquid Mixtures in Cooling Applications

2.1Refrigeration

A widely used cooling process is refrigeration. The term refrigeration refers to the process of removing heat from a system and transferring it to a region where it has no effect. Refrigeration is used to lower the temperature of the system, and then maintain this decreased temperature. There are three main forms of refrigeration: Cyclic refrigeration, non-cyclic refrigeration and thermo electric refrigeration. Non- cyclic refrigeration does not depend on mechanical work for the process to proceed, the refrigeration depends on a substance changing state by absorbing heat from the surroundings, therefore increasing the temperature of the refrigerant and decreasing the temperature of the surroundings. This method of refrigeration is usually found in short term applications such as using ice to cool stock at markets or subliming carbon dioxide in a laboratory experiment. Here different mixtures of substances can be used to achieve different overall operations such as using an ice slurry to reduce the temperature to around 0C for a period of time, however by using ice with sodium chloride the cooling temperature can be decreased to -20C but for a shorter period of time. Other mixtures include ice and ethylene glycol, propylene glycol, sucrose, glucose and ethanol. This increased cooling effect is due to the melting enthalpy contributing to greater heat absorption than in single phase coolants [1].Thermo electric refrigeration creates a heat flux between two different materials, then transfers heat against the temperature gradient, therefore reducing the temperature of the surroundings. This process requires no working fluid, it is a purely solid state operation so will not be included in this report. Cyclic refrigeration is the most common form of refrigeration, it consists of a refrigeration loop where heat is removed from the system and transferred to atmosphere through the use of several working fluids. Two main cycles exist: Vapour compression cycle and vapour absorption cycle. Vapour compression cycle operates by compressing the working vapour, then condensed to a liquid, passing this liquid through a throttling valve giving a mixture of liquid and vapour and finally evaporating it. After the evaporation stage the vapour is passed back into the compressor for the cycle to start again. Vapour compression is the most widely used refrigeration method; it is used in large and small scale processes, in domestic and industrial applications. These processes include: air-conditioning units in buildings and cars, refrigerators and freezers, some cooling of electronics, chemical separators and liquefaction processes. Vapour absorption refrigeration has a comparable cycle to the compression cycle, except from the process of increasing the pressure of the working vapour. The absorption system contains an absorber which dissolves the refrigerant into a liquid, then a pump raises the pressure and a generator removes the refrigerant vapour from the high-pressure liquid. Work is required by the liquid pump, however it is much smaller than required by the compressor in the vapour compression cycle. It is important to operate suitable combinations of refrigerants and absorbents in order to maximise efficiency. There are a wide range of applications for cooling fluids and refrigerants in which there is large interest: air-conditioning units in buildings and cars, refrigerators and freezers, cooling of electronics, nuclear reactor cooling, cooling streams on many chemical plants, separators, liquefaction processes. Many of these applications use substances that are rapidly becoming unfavourable such as chlorofluorocarbons (CFCs) which are being phased out and will be totally replaced by 2020 in developed nations. The widely used interim replacement for CFCs are hydrochlorofluorocarbons (HCFCs) due to their reduced ozone depletion potential, typically 1-10% that of CFCs, and hydrofluorocarbons (HFCs) due to their 0% ozone depletion potential. However there are significant drawbacks in using these as replacements. HCFCs cannot be considered long term alternatives as they still contribute to ozone depletion and are expensive. Despite having no impact on the ozone HFCs are not ideal for long term use as they are more expensive that CFCs, are difficult to work with, offer reduced performance and have a short lifespan [2]. To regain some of the favourable properties that CFCs had, mixtures of components are being studied to see whether a compromise of two or more refrigerants can give similar results to that of chlorofluorocarbons. It would be particularly beneficial if a mixture could be formed for use in an existing refrigeration system as this would mean only the working fluid would need to be replaced, leaving the existing mechanical process untouched.

Dichlorodifluoromethane (R12) was used extensively in domestic refrigerators, however since its ban in 1994 mixtures of hydrocarbons and hydrofluorocarbons have been investigated as possible replacements. One possible mixture is that of propane, butane and 1,1,1,2-Tetrafluoroethane (R134a). Experimental research shows that ternary mixtures of butane/propane/R134a can be just as successful for domestic refrigeration as R12. A number of tests have been performed at a range of evaporator duties (100 to 350 W) in a domestic refrigerator whilst varying the composition of the refrigerating mixture in between trials. It was found that a mixture of propane/butane/R134a with mass fractions of 31.25/31.25/37.5 (%) could successfully be installed in a domestic refrigerator originally designed to work with R12, without modifying any part of the refrigeration apparatus. This mixture gives performance characteristics very similar to those of R12. The coefficient of performance at a 100 W evaporator duty is 5.4% less than that of R12, and 0.8% less at a 350 W duty [3]. These differences are due to the slightly higher compression power requirements of the mixture but are negligible when considering a relatively small refrigeration unit. An increased volumetric efficiency for the compressor is also noticed when operating with this mixture. Applying this mixture to larger scale operations is not possible [3] as the larger the evaporator duty becomes, the further the coefficient of performance deviates from the initial refrigerant causing severe efficiency losses on industrial scale applications.

The majority of HFCs have high global warming potentials (GWP) this means that although they do not deplete the ozone, they do contribute to global warming and therefore cannot be considered for long term use. As hydrocarbons are an environmentally acceptable alternative and inexpensive, their use as a refrigerants in domestic refrigerators is very attractive. Mixtures of hydrocarbons are presently being used as a replacement for both CFCs and HFCs. Recent experiments have been conducted using LPG (30% propane, 55% n-butane and 15% isobutene by mass). These experiments have concluded that by altering the masses of refrigerant, an optimum working mass can be found. For LPG it was found that a mass of 80 g/100W evaporator duty gives the best performance relative to R12 and R134a. However if the number of components present is reduced to two, then the refrigerator operating with propane/butane (60%/40% by mass) needs less external energy to be supplied compared with a refrigerator operating with propane/isobutane (60%/40% by mass). This is due to the latent heat of the propane/butane mix being higher than that of the propane/isobutane. Therefore, the mass of propane/butane is lower than that of the propane/isobutane at 70g/100W and 75g/100W respectively [4]. This means the duty of the compressor is reduced, leading to reduced overall energy consumptions, whilst still transferring the same heat duty as the other mixtures.

Aside from the mixtures used, the flow boiling heat transfer of mixtures has a large influence on the success of the techniques being used [5]. Extensive experimental results show that when using mixtures as the working fluid a direct comparison with pure components is difficult due to the effects of mass diffusion between the components in the mixture[6][7]. Experiments of flow boiling with a mixture of bromotrifluoromethane/ difluoro-1,1-ethane (R13b1/R152a) in a horizontal tube have shown that local and average heat transfer coefficients for the mixture are significantly lower than for either pure component [8]. Further work has shown that with the R13b1/R152a mixture having composition of 0.07, 0.22, 0.36 and 0.64 mole fraction R13b1, there is a substantial loss of heat transfer coefficient with increasing R13b1 mole fraction. However the most unusual trend found during this experiment was that a circumferential variation of wall temperatures was noticed with mixtures in an annular flow regime. During annular flow, heat is conducted across the liquid layer, the liquid film at the bottom is thicker than that at the top due to gravity. As a result, for pure components the wall temperature at the bottom of the tube was higher than at the top because of the increased resistance to heat transfer conduction, resulting in a lower heat transfer coefficient at the bottom of the tube. For mixtures, new traits were observed; wall temperature at the bottom of the tube was lower than the top. This behaviour is expected to arise from composition variations around the circumference of the tube. Heat transfer coefficients for mixtures were constant over the composition range of 0.1- 0.64 mole fraction of R13b1 [9]. However, large discrepancies were seen in the locations of heat transfer.

Further studies of flow boiling heat transfer have enlightened new characteristics of pure refrigerants and a non-azeotropic refrigerant mixture in horizontal tubes. A series of tests were carried out for pure and mixed refrigerants of chlorodifluoromethane (R22) and 1,2-dichlorotetrafluoroethane (R114) at different compositions. Results found that nucleate boiling was totally removed for mixed refrigerants and most heat transfer occurred during the convective evaporation region. In this region heat transfer coefficients of mixtures were almost 36% lower than the expected ideal values. Variations of wall temperature when using mixtures arose from composition variations of up to 0.07 mole fraction between the top and bottom of the tube, causing the corresponding circumferential variations [9]. In addition, flow patterns change with composition, leading to uneven heat transfer [10]. Flow boiling of pure refrigerants and refrigerant mixtures in horizontal tubes have also been investigated. Mixtures of difluoromethane/ tetrafluoroethane (R32/R134a), propane/isobutene (R290/R600a) (non-azeotropic refrigerant mixtures) and difluoromethane / pentafluoroethane (R32/R125) (azeotropic refrigerant mixture) were used as the working fluids. These mixtures have zero ozone depletion potential and are used to replace CFCs and HCFCs. It was concluded that heat transfer coefficients depend strongly on heat flux in the laminar flowrate region [11].

Boiling of mixtures differs substantially from that of pure fluids due to a number of factors such as the effect of composition on nucleation [12], significant changes in physical properties of mixtures with composition [13] and the reduction of vapour-liquid exchange and evaporative mechanisms [14]. The heat transfer coefficient of mixtures is generally lower than that of the equivalent pure fluid with the same physical properties. Theoretical models suggest that the mixture affects nucleate boiling and the thermal resistance in the vapour phase, therefore causing a decreased heat transfer coefficient. Using mixtures of refrigerants causes the heat transfer coefficient of the mixture to be significantly lower than that of pure fluid in the boiling-dominant region, but is almost equal in the convection-dominant region. This is due to the effect of diffusive resistance being small in the convection-dominant region [15].

2.2 Micro-applications.

In recent years there have been many technological advances in the micro electronics industry, this has lead to an increase in density and speed of electronic chips and therefore an increase of dissipated heat flux. New high powered electronic chips can potentially generate heat fluxes of up to 100W/cm2. As existing cooling systems cannot dissipate this flux effectively, there is a build up of temperature within the electronic device and premature device failure can occur along with compromised safety. It has been proposed to cool the electronic chip with a liquid mixture in order to dissipate this heat more efficiently, therefore it is important to understand the boiling and two-phase flow of mixtures in small scale systems. Many advanced technology and traditional industries such as electronic, pharmaceutical and medical industries are also increasing their dependence on micro-scale thermal, fluid and chemical systems for increased quality of product and increased efficiency of the process e.g. applications of compact evaporators in hydrocarbon separation, liquefaction of gas (nitrogen, helium, natural gas) and the separation of oxygen and nitrogen. This means total reengineering of the process is required including micro-pumps, compressors, turbines and heat exchangers as well as the working fluid within the cooling systems. Although this technology is still young, much research has already been carried out. Now particular areas of interest are in understanding flow patterns on small scale applications, flow boiling heat transfer, critical heat flux (CHF) and two phase flow pressure drop. Significant differences of transport phenomena between small diameter channels and normal size channels exist, so it is important to distinguish between them to prevent confusion. A unanimous agreement has not been established yet, so for the basis of this report the hydraulic diameter will be used to define the category of scale. The transition between small and normal size channels will occur at a hydraulic diameter of around 7mm. The most common two-phase flow and flow boiling of mixtures application is within capillary tubes in refrigeration, heat pump and air-conditioning systems. A capillary tube is a constant area expansion device used in small scale systems. This tube usually has an inner diameter of between 0.5 to 2 mm and a length ranging from 400 to 2500 mm. A vast quantity of data for an adiabatic capillary tube with use of CFC and HFCs has been compiled. However, a search for alternative refrigerants has become an important region of investigation. Refrigerant mixtures have been chosen to replace these unwanted refrigerants in small scale applications. Engineers now must research the performance of capillary tubes using alternative refrigerant mixtures in order to correctly design new refrigeration equipment. This will include determining appropriate length and diameter of the capillary tubes at given refrigeration capacities and operation conditions. It is, therefore required to understand the process of gas-liquid two-phase flow and flow boiling of new refrigerant mixtures in capillary tubes. Limited study has been reported under these conditions, although there are a lot of studies of two-phase flow and flow boiling of pure fluids in small and mini channels. One possible reason for this is due to the complexity and difficulty of studying the two-phase flow and flow boiling phenomena of mixtures in small and mini channels. The research that is present includes two-phase flow frictional pressure drops, mass flow rates, qualities and temperatures. Experimental and theoretical models have been created to provide a basis for the design and operation of capillary tubes in working systems. Experiments of flow boiling using water/methanol binary mixtures, in horizontal micro channels with hydraulic diameter between 0.133 to 0.343mm have investigated. The "influence of liquid compositions of the more volatile liquid on the heat transfer performance", and the "flow boiling phenomena in the micro channels" were of particular interest. These experiments found that during the boiling of binary mixtures, as the component concentrations in the liquid and vapour phase change along the test section, the local bubble-point temperature rises as the heavy component builds up the liquid phase [16]. Thus not only must latent heat be added to the fluid to evaporate it, but also sensible heat must be added to both phases to heat them up to the new local bubble-point temperature[17] . Studies of flow boiling of mixtures in plate heat exchangers with small hydraulic diameter channels have also been presented. Boiling heat transfer and frictional pressure drop of the mixture of difluoromethane (R32) and pentafluoroethane (R410a) in a heat exchanger with a hydraulic diameter of 6.6 mm were examined. It was found that both boiling heat transfer coefficient and frictional pressure drop increased linearly with heat flux. In addition, the refrigerant mass flux has significant effect on the flow boiling heat transfer coefficient at high heat flux. It was also noted that refrigerant pressure has very slight influence on the heat transfer coefficient [18].With the progress of new technologies related to this area, there is an incentive to further research two-phase flow and flow boiling of mixtures in small and mini channels. For both theory and practical uses, current deficiencies in knowledge could be recovered by exploring:

(i) Two-phase flow and flow boiling of mixtures over a wide range of channel diameters in the micro scale.

(ii) Investigating the effect of mixture composition on heat transfer coefficient and two-phase pressure drop.

(iii) Flow patterns and transition regions of two-phase flow and flow boiling of mixtures in micro-channels. [15]

3.0 Azeotropic, Near-Azeotropic and Zeotropic Mixtures

Azeotropic mixtures are mixtures that behave like a pure fluid, i.e. under constant pressure they will condense or evaporate at a constant temperature and the composition in the vapour and liquid phases will remain the same. This means that any refrigerant leak from the system will not change the composition of the remaining refrigerant, meaning that the refrigeration system will continue to operate as normal. Figure 1 shows that at the azeotrope point there is only one possible composition of the mixture at corresponding pressure. The composition at which an azeotropic mixture exists is a function of temperature, therefore no azeotropic mixtures can fully exist in refrigeration [21]. However, the azeotropic mixtures used have a very small composition range therefore can be treated in the same way as a true azeotrope. Successful use of azeotropic mixtures has lead to wide use in refrigeration.Near azeotropic mixtures are mixtures that have small temperature variations during phase change, and little difference in composition in the vapour and liquid phases at equilibrium. Due to this fact, use of near azeotropic mixtures have only been used to give a wider range of refrigeration possibilities, their use is not ideally suited to cooling and refrigeration applications [19]. An example of this is the addition of small amount of propane to R502 to increase its solubility in the lubricating oil [21].Non-azeotropic (zeotropic) mixtures have different vapour and liquid compositions when in equilibrium. From Figure 1 it can be seen that there is a wide range of possible compositions between the dew and bubble point curves, to the left and right of the azeotrope point. This is the result of non-similar boiling points of the individual components that make up the zeotropic mixture [20]. During phase change different boiling points of the individual components leads to different evaporation, or condensation temperatures relative to the other components. This leads to a change in the composition of the mixture, therefore changing the dew and bubble point temperatures of the remaining mixture [21].

4.0 Flow Characteristics

4.1 Temperature Glide

A non-isothermal phase change process occurs within the dew and bubble point temperatures known as the temperature glide, the magnitude of this temperature glide is dependent on the boiling points of the components and the composition of the mixture. Temperature glide decreases with increasing pressure.

Mixtures with significant temperature glide, greater than 5C, in theory have the potential to improve performance and energy efficiency of vapour compression systems [21]. In order for these improvements to take place the temperature profiles of the refrigerant and the heat transfer fluid must be coordinated. This allows a fairly constant, small temperature difference between the two different streams. This process is known as glide matching, it reduces the possibility of reverse heat transfer; thereby improving the performance and energy efficiency. Temperature glide matching can be analysed by interpolating from the Temperature-Entropy graph for the refrigerant. Perfect glide matching is only achievable in some heat exchanger set-ups, possible configurations include: shell and tube, annular tubes and flat plate heat exchangers. It is not possible to match the temperatures in micro heat exchangers as they do not fully comply to counter flow conditions.

Advantages of glide matching are:

(i) The compressor will be operating under a reduced pressure range, improving performance,

(ii) Higher stream temperatures can be achieved using appropriate mixtures, using less compressor work. [21]

To have a successful temperature glide match, an adequately sized heat exchanger is needed. Smaller temperature differences require larger heat transfer areas to transfer the same heat capacity. The right combination of heat exchanger size, heat transfer fluid and refrigerant contribute significantly to the actual performance of temperature glide matching. If the cooling system requires use of a linear refrigerant and a heat transfer fluid, glide matching can only be achieved by altering the heat transfer fluid flow rate [22].

Looking at Figure 2, a large gradient represents a higher mismatch in glides that leads to poor performance.

4.2 Circulating composition

When a single component is used in a vapour compression system, the concentration of the refrigerant throughout the system is equal. However, when zeotropes are being used composition variations are noticed [23] [24]. This difference is thought to arise from internal fractionation of zeotropes [24]. Fractionation is the result of preferential boiling (or condensation) in the mixture components. During the boiling process the vapour phase gains the more volatile component from the mixture while the liquid phase loses this component. This causes the less volatile components to be held up causing the mixture composition within the system to become different than that of the original. The shift in circulating composition leads to altered system pressures, temperatures, capacity and energy efficiency [25] [21].

4.3 Condensation

The condensation heat transfer phenomenon for mixtures is fairly complex. Investigations into the effect of mixture composition on heat transfer coefficient have not shown any recognisable pattern [26]. However it is known that stratification of the composition and the temperature in the vapour phase reduces the heat transfer coefficient [27]. The effects of stratification decrease as vapour velocity increases.

During condensation the low volatility components diffuse into the vapour away from the vapour-liquid interface. This mass fraction gradient causes a thermal gradient and mass transfer resistance in the vapour diffusion layer, affecting the condensation. This influence depends on the volatility difference between the mixture components. Furthermore, in forced convection the composition of the mixture, the shape of the surface and the velocity of the vapour influence the heat transfer coefficient significantly [21].

4.4 Boiling and evaporation

Using more than one component for mixture refrigerants changes the required superheat to initiate and sustain nucleation [12]. The change in superheat is the foremost and most complex contributor to the heat transfer coefficient depletion of mixtures [28]. Bubble growth during nucleation is limited by the liquid bordering the bubble surface becoming enriched with the more volatile component. For a binary mixture, the minimum heat flux increases with the presence of an additional component. This shows that more heat transfer and a larger wall superheat are required to initiate the boiling process [22]. These cause considerable reduction in nucleate boiling heat transfer. The heat transfer coefficient of nucleate boiling is much lower than that of a pure refrigerant with similar physical properties. This decrease in heat transfer coefficient increases with pressure and does not change with composition [29]. The heat transfer coefficient deterioration is mainly due to a rise in the local boiling point temperature due to evaporation of one of the components during bubble formation and growth [30]. In addition, the mass transfer resistance resulting from concentration difference between the vapour and liquid phases aggravates the heat transfer deterioration [31].

As with pool boiling, forced convective heat transfer of mixtures is fairly poor. It is thought that the mixture heat transfer coefficient is a function of the overall composition, and the physical properties account for about 80% of the heat transfer reduction seen in mixtures. In addition, the effect of mass flux has a large influence on the heat transfer coefficient due to changes in mass transfer resistances caused by depletion of the more volatile components [21].

4.5 Void fraction.

The most significant variable in two-phase flow and heat transfer is the void fraction of the vapour phase in the cross-section of the flow channel. The void fraction is regarded as the volume lost to the vapour phase within a system. Four main vapour flow regimes exist: bubble, slug, annular and stratified flow. Under bubble flow the majority of the working fluid is liquid, however small bubbles form and flow at a roughly equal flowrate to the vapour phase. This regime does not have a significant impact on the overall efficiency of the system, however it can cause problems for the hardware i.e. pumps. Slug flow has a larger vapour phase within the tube, as the two phases move at different flowrates uneven flow occurs as seen in figure 3. Annular flow has the majority of the vapour phase travelling in the centre of the tube and liquid on the walls of the tube. This regime requires a large vapour flowrate and can have very significant effects on both efficiency and the system operation. Stratified flow has roughly constant low flowrates for both liquid and vapour phases, which causes the flow to separate into two distinct laminar flows as seen in figure 3. Stratified flows are most common in the cooling processes, however they are just as undesirable as annular flow. As the vapour and liquid phases do not travel at the same velocity the void fraction and dynamics of the two-phase interface are a function of the flow itself [32]. This void fraction decreases the heat transfer coefficient, leading to decreased overall efficiency. Void fraction is a much larger problem in micro applications as the vapour bubbles formed usually lead to slug flow which can decrease the efficiency by up to 40% [32].

4.6 Flow Patterns

During practical applications major design complexities arise when dealing with two-phase flow. Mass, momentum, and energy transfer rates are quite sensitive to the geometric distribution of the system. Geometry can strongly affect the interfacial area available for mass, momentum or energy transfer between phases. Different flow patterns mainly occur as a result of geometry, volume flow and components in the mixture. The boundaries between these flow patterns occur because one flow regime becomes unstable as the boundary is approached, this instability increases and causes transition to another flow pattern [33]. These phase transitions are unpredictable as they depend on many other variables of the flow. This means that flow pattern boundaries are never distinguished lines but poorly defined transition zones.

Figure 4 shows the rough boundary regions of two phase flow for a water/air mixture. Marked on the diagram are flow regions i.e. low liquid mass flowrate and high vapour flowrate would lead to wave flow, or high flowrates of both liquid and vapour would lead to slug verging on annular flow. The hatched regions are practical regime boundaries and solid lines are theoretical predictions for phase boundaries. Many different forms of this chart exist, and will change for flowrates and for component mixture.

5.0 Advantages of Using Mixtures

Mixtures offer the advantage of tailoring the composition to suit various temperature requirements of the system. By choosing a mixture with suitable characteristics for the application greater heat fluxes can be dissipated, smaller units can be installed and therefore greater overall efficiencies will be noticed. It is also possible to control the properties such as toxicity, flammability, oil miscibility by manipulating the composition. If a zeotropic mixture is chosen then, vapour compression cycles can be improved by choosing a suitable temperature glide. Hence, mixtures are finding greater use.

6.0 Disadvantages of Using Mixtures

The advantages brought forward with mixture use also bring forth several disadvantages. If a leak occurs within a system the composition of zeotropic mixtures will be changed, thus changing the way the cooling or refrigeration system operates, therefore reducing efficiency. If the heat duty of the application causes only one component in the mixture to evaporate, then the efficiency of the whole system will be disturbed. This is due to the heat transfer coefficient of the vapour being less than the liquid and therefore causing uneven fluid flow. A further consequence of evaporation or boiling in the system will be that of increased pressure, if the system is closed this will have safety consequences as well as operational consequences. Flammable refrigerants are gaining increased use and it is believed that servicing of such systems is a potential safety hazard, it is therefore necessary to educate the operators in the correct maintenance procedures.

7.0 Conclusion

The most effective substances used as the individual components in a cooling mixture are; readily available in pure form, stable, non-hygroscopic and non-aggressive. This means that once combined to form the operating working fluid mixture the components will not react with any other substance they may come in contact with and will not attract water molecules from the surroundings. These characteristics ensure a fairly predictable operating system and one which can be operated for prolonged periods of time without excessive maintenance. It is also important to ensure that the azeotropic mixture composition should change very slightly with temperature if temperature glide has not been accounted for in the design stage of the cooling system. The defining characteristic of a minimal temperature glide is that the molar evaporation heats of the individual pure substances forming the azeotropic mixture will be of similar magnitude. The relative independence of the vapour composition from temperature in the azeotropic mixture is very important and must be considered before using as the working fluid. In order to assess the level of independence, gas chromatographic procedures should be used. Concentration of the vapour phase at equilibrium should be calculated at various temperatures in the azeotropic mixture [36]. For large and normal scale operations, it is particularly beneficial to utilise mixtures in place of pure components as this allows for greater operating efficiencies to be found, it is also possible to model the whole system in practical and theoretical terms as the process is fully understood. As for small diameter channels, no correlation and heat transfer mechanisms are available. Even for the experimental studies of flow boiling heat transfer available in the literature two main limits exist. Firstly, some data reduction methods are not appropriate. The other is that there are limited test channel diameter range for both small and micro dimensions [15]. In all, there lacks a systematic knowledge of two-phase flow and flow boiling of mixtures in small and mini channels. However, it is still a long-term task to achieve an understanding in this important area because of the complexity and difficulty of two phase flow and flow boiling phenomena of mixtures in small and mini channels. Efforts should be made to contribute to both experimental and theoretical studies in this important area in the future.

8.0 References

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