Numerical analysis of a two-phase injection refrigeration cycle using R32

The present paper reports the performance of a popular refrigerant R32 (Difluoromethane, CF2H2) experiencing the two phase injection process. Two phase injection process may lower the discharge temperature of a multistage compressor. In order to investigate the role and impact of two-phase injection on a compressor, a Scroll compressor is selected because scroll compressor has high tolerance for liquid refrigerant. A reputed compressor is chosen where all the operating conditions and specifications are available in public domain. The modelling and analysis of refrigeration system is carried out using a simple MATLAB code. Around 200 iterations were performed for four different condensing and evaporating temperatures. The maximum reduction in discharge temperature is found to be 44°C when compared to R410A used in the same system. Cite this article as: Praveen A, Debjyoti S. Numerical analysis of a two-phase injection refrigeration cycle using R32. J Ther Eng 2022;8(2):157–168. Journal of Thermal Engineering Web page info: https://jten.yildiz.edu.tr DOI: 10.18186/thermal.1077857


INTRODUCTION
Since the first vapor compression refrigeration system was made by Jacob Parking in 1834, a large number of chemical substances have been tried as working fluid like ammonia, methyl chloride, carbon dioxide etc. [1]. After the invention of dichloromethane in 1930 by Thomas Midgely and Albert Henna chlorofluorocarbons (CFCs) and hydrofluorocarbons (HCFCs) became the dominant types of refrigerants. R11 and R12 were the most widely used refrigerants [1]. When HVAC industry started to grow at a faster rate, the production of these refrigerants exceeded many million tons, the high production of these refrigerants was seen as a cause of concern as these refrigerants were found responsible for ozone depletion and global warming [2]. Due to environmental concerns the production of R11 and R12 has been stopped. This led to their replacement with R22. But as R22 is also responsible for ozone depletion many R22 alternatives were introduced into the market such as R410A, R407C etc. which are non-ozone depleting refrigerants but have very high global warming potential (GWP). For example, R410A has a GWP of 2088 on a 100 year time frame (IPCC, 2007) [3], also needs to be phased out. Now, the best alternative for R410A is R32 which is a non-ozone depleting refrigerant with global warming potential one third of R410A (GWP = 675 on a 100 year time frame) [4]. The pressure ratio of R32 similar to R410A [5]. Taira et al., [5] suggested that among various refrigerants R32 can be retrofitted immediately. In order to gauge the performance potential of R32, Lemmon et al., [6] did the thermodynamic assessment of R32 and R410A in ideal vapor compression cycle using REFROP. They found that R32 has 10% more volumetric efficiency than R410A and refrigeration capacity per mass is 52% greater than R410A. They also found that discharge temperature is 13°C higher than R410A. Compressor discharge temperature of R32 seems to be high in regular compressor so the present work proposes the use of two-phase injection in a scroll compressor. Numerical simulation is performed using NIST REFROP data to calculate discharge temperature. A simple MATLAB code links simulation with NIST data [7].
In order to decrease the discharge temperature of refrigerants various method can be used such as vapor injection, liquid injection and two-phase injection etc. [8]. Vapour injection Cycle has been studied by many researchers.
Wang et al., tested the flash tank at -18°C and found that maximum COP improvement to 23% [9]. Currently popular refrigerants R22 and R410A are going to be phased out in few years from now. Recently Xu et al., did performance comparison of R32 and R410A in vapour injection cycles and concluded that the capacity and coefficient of performance (COP) improvements using R32 can reach up to 10% and 9%, respectively and R32 is the best alternative to replace R410A in terms of performance [10]. System performance for R32 can be further enhanced by component optimization using an identical cycle that uses R410A.
Refrigeration cycle working with evaporator temperature 1 to 8°C and a condenser temperature ~65°C should be given importance as they are practical values of summer air conditioning. Simulation work related to two-phase injection cycle suitable for air conditioning application is rare because there are multiple mathematical equations involved [11]. Simulation of Two-phase injection refrigeration cycle employing refrigerant R32 is worth investigating as R32 is expected to dominate the Air conditioning market in this decade. The main aim of the present work is to analyze the effect of two-phase injection in decreasing the discharge temperature of R32 and its effect on the COP of the refrigeration system with a scroll compressor.

Performance of Refrigeration System
Before simulating two-phase injection thermodynamic assessment of R32 is done in ideal refrigeration cycle using actual operating conditions obtained from the compressor manufacturer (Model YP292K1T-TND, R32 COPELAND SCROLL TM 3-50) [12]. A sample calculation can be interesting. A popular scroll compressor was chosen for doing this calculation (Table 1-3) which is used for air-conditioning. The operating conditions are taken from the manufactures webpage. However the operating conditions which  the manufacturers of the compressors use are under standard test conditions. These are common to test any Airconditioning system. The manufacturer has also provided the mass flow rate of R32 and cooling capacity of the system at various operating conditions. Isentropic efficiency (ratio of work input to isentropic process and actual process) [11] is given in Eq. 1.
Cooling capacity is the system's ability to remove heat [11] and given in Eq. 2.
Coefficient of performance and energy efficiency ratio are important parameters [11] and given in Eq. 3 and Eq. 4.

COP =
Refrigeration effect Work done EER is the ratio of cooling capacity in BTU per hour and electrical energy input in watt, Therefore referring to Table 2 (Table 1) which is similar to calculated value of 7.31. The value for isentropic efficiency is also available for different operating conditions. The discharge temperature of R32 is 137.6°C for given working condition therefore steps should be taken to lower the discharge temperature of R32 for the safe working of the compressor. High discharge temperature often leads to the compressor failure due to the degradation of the compressor oil [13,14]. Volumetric efficiency and isentropic efficiency of the compressor affect the performance of any refrigerator or air conditioner. Volumetric efficiency controls the refrigerant flow rate that will pass through the compressor. Low volumetric efficacy eventually decreases the mass flow through the evaporator thus decreases the cooling capacity of the system. Volumetric efficiency is inversely proportional to the pressure ratio. Due to low volumetric efficiency at high pressure ratio, the cooling capacity decreases as the pressure ratio increases further. However isentropic efficiency affects the COP of the refrigeration systems by affecting the work done by the compressor. Table 4 comprises of change in cooling capacity, power requirement, mass flow rate, EER and isentropic efficiency of the same compressor at  The cooling capacity, isentropic efficiency, mass flow rate at operating condition 1 and at operating condition 4 are low though the power rating is continuously increasing through conditions 1 to 7. This shows that as the pressure ratio increases system performance decreases. The decrease in mass flow rate is due to the decrease in volumetric efficiency as the pressure ratio increases [15]. The decrease in EER of the system is due to decrease in isentropic efficiency of the compressor at higher pressure ratio [8]. The power required by the compressor increases as the pressure ratio increases. The sample calculation was done on operating condition 4.

Two-Phase Injection System
In a two-phase injection refrigeration system a two stage compressor is used ( Figure 1). The refrigerant (mass flow rate = m 1 ) after getting compressor at first stage gets mixed with the low-temperature two-phase refrigerant (mass flow rate = m 2 ) coming out from first expansion device. The refrigerant mass flow rate after getting mixed (mass flow rate = m 1 + m 2 ) gets into the second stage of compressor and is compressed till condenser pressure [16][17][18]. The refrigerant after getting condensed enters the first expansion device where the refrigerant is depressurized till the discharge pressure of first stage of compression ( Figure 2). Refrigerant passing through the first expansion device splits into two-parts. One part (m 2 ) gets into the injector and other part gets into the second expansion device where the refrigerant is further depressurized till evaporator pressure. A two-phase injector can be used to inject the refrigerant coming out of the first expansion device to the compressor at the end of first stage. The mixing of refrigerant decreases the temperature of the refrigerant vapor. A mathematical model can be developed to predict the discharge temperature, COP of the system. This mathematical model of vapor compression refrigeration system with two-phase injection system can be coded in MATLAB to recall the parameters of REFPROP.

MATHEMATICALMODELLING OF TWO-PHASE INJECTION SYSTEM
Compression pressure ratio (P r ) is an important factor in design of the compressor. The present work involves twostage compression, so there will be two compression ratios for two stages [11]. However, the pressure ratio of both the stages can be same or different. The pressure ratio of first compressor is given as P r1 and pressure ratio of second stage is given as P r2 ranging from 0 to 6.
In addition the pressure which is important in calculation of pressure ratio of 1 st stage of compression is called injection pressure ratio [11] which is defined as shown in Eq. 7.
r p can vary from 0.001 to 0.999. If r p = 1, P inj = P 2 and twostage cycle will then become single stage and 0 < r p < 1.
Optimization of r p is another objectives of this work. It decides the pressure at which injection will take place visa-vis the discharge pressure of first stage of compression.  Pressure Enthalpy Chart for two-phase injection [19].
Another important parameter with two-phase injection system is the mass flow ratio of injection and it is given in Eq. 8 [11].
m mass of injection total mass flow rate r = Here m r can vary from 0.001 to 0.999. If m r will become zero then in that case no refrigerant will be injected to the compressor at an intermediate stage and the cycle will become single stage system instead of two-stage. If the value of m r becomes 1 then no refrigerant will get into the second expansion device and no-refrigeration effect will be produced at the evaporator. Optimization of m r is also an objective of this work.

First Stage of Compression
Inlet condition to the first stage of compressor is represented in Table 5. The injection pressure P inj can be calculated after selecting certain value of pressure ratio, r p . Pressure P 2 for the refrigerant must be found at T = T cond and x = 0. Using Eq. 7 injection pressure can be found; P inj = r p × (P 2 -P 1 ) + P 1 and P 6 = P inj . Enthalpy h 6s must be found at P = P inj ; S 6s = S 1 .
First stage compressor efficiency as shown in Eq. 9 [19].
Temperature T 6 can be found at P = P inj and enthalpy h = h 6; Work input at first stage compressor [19] = m 1 × (h 6 -h 1 ) Work input at first stage compressor [19] = m (h h )

Mixing of Fluids
Injection mass flow ratio m r varies from 0.001 to 0.999, m inj can be calculated after specifying the value of m r . Therefore refrigerant injected m inj = m 2 = m r × m total that follows the mass balance equation, m total = m inj + m 1

Energy Balance
From energy balance concept if m t is the total enthalpy then, m 1 × h 6 + m inj × h 5 = m total × h 7 Therefore enthalpy after first stage (Figure 2), h 7 (12) Saturation temperature T 5 can be be found at P = P inj and x = 1; whereas T 7 to be found using the relation h = h 7 . As per COPELAND compressor requirement minimum degree of superheat at the entry of compressor should be 11.11°C; T sup = (T 7 -T 5 ) ≥ 11.11°C.

Second Stage of Compression
Pressure ratio r p2 can be obtained using Eq. 6; now, isentropic efficiency of second stage compression can be found using Eq. 13 [19].
Where second stage compressor temperature T 2 corresponds to P = P 2 , h = h 2 and work done at second stage = m total × (h 2 -h 7 )

Condenser
Sub cooling happens in the condenser and the temperature [19], Where pressure P 3 = P 2 and h 3 = Enthalpy corresponds to P = P 3 , T = T 3 .

First Expansion Tube
Use the energy balance concept to refrigerant flow [19], Where pressure P 5 = P inj and T 5 = Temperature corresponds to P=P inj , h=h 5 .

Second Expansion Tube
Similarly use the energy balance concept to refrigerant flow [19], m h m h Where pressure P 4 = P 1 and Temperature = T 4 corresponds to P = P 1 , h = h 4 .

Evaporator
Use Eq. 2 to find refrigeration effect and use Eq.3 and Eq.4 to get COP and EER respectively.

Pre-validation
Isentropic efficiency is a function of pressure ratio so relationship between isentropic efficiency and pressure ratio should be established for the compressor. Isentropic efficiency of the compressor is provided by the manufacturer according to various pressure ratios and operating condition [12]. The data of isentropic efficiency of the compressor is plotted against the pressure ratio to obtain a relationship between the isentropic efficiency and pressure ratios. Figure 3 shows the relationship between the pressure ratios of the Copeland compressor. Isentropic efficiency is taken along y axis and pressure ratio along x axis. Now the isentropic efficiency of the two stages of the compressor can be expressed as polynomials given in Eq. 17 and Eq. 18 respectively.
Interestingly, experimental isentropic efficiency is 0.705 whereas isentropic efficiency with best fit graph is 0.7069; thus it pre-validates this model (Eq.1 to Eq. 18). The mathematical model is coded in MATLAB. In order to test the accuracy, the EER of experimental cycle is compared with the EER obtained from the code. Values are as shown in table 6.
In order to convert two-phase injection cycle to single stage cycle r p is considered as 0.9999 and m r as 0.0001 respectively. The percentage difference between the experimental EER and EER obtained from MATLAB code is ~0.4%. The difference is due to the fact that isentropic efficiency used in the MATLAB code is obtained from best fit equation (Eq. 17 and 18) which predicts the isentropic efficiency value with ~98.1% accuracy.

RESULTS AND DISCUSSIONS
After validating the COP/EER value with COP obtained from MATLAB code, various values of r p and m r are selected ranging from 0.001 to 0.999 to simulate four different cases with varying condenser and evaporator temperature.

Case 1: T cond = 60°C and T eva = 1.66°C
The variation of COP with respect to the pressure ratio for mass injection ratio m r = 0.05, 0. The value of COP increases as mass injection ratio increases [11]. The orange line in Figure 4 represents the COP of single stage cycle (COP = 2.1491) without two-phase injection. The COP obtained in said condition are 5%, 4.38%, 3.54% and 2.67% lesser than COP of refrigeration system of same capacity but using a single stage compressor.  Figure 5 represents the discharge temperature without two-phase injection. The discharge temperature without two phase injection is as high   [24]. Refrigerant in any air conditioner evaporator will be at <7.22°C.
The minimum discharge temperature obtained at the r p 0.34 for said values of m r are 123.68°C, 110.37°C, 103.42°C and 96.31°C respectively as shown in Figure 8. The degree of superheat should be maintained for mass injection ratio as 137.02°C. It's safe to operate R32 at low discharge temperature as R32 is slightly inflammable [20,21]. Vali et al., reported a discharge temperature of 110.18°C with COP value of 4.245 where the condenser and evaporator temperature assumed to be 54.4°C and 7.2°C respectively (superheat temperature = 10°C, Sub cooling 5°C) [22]. As per scroll compressor specification, R32 will not work in this condition [11]. Pramudantoro et al., tested and reported maximum COP for R32 is 2.11 for a 0.75 TR air conditioner [23].
Variation of degree of superheat at the inlet of second stage of compressor with respect to 50 different values of r p ranging from 0.001 to 0.999 can be plotted in a graph. The degree of superheat should be near to 11.11°C (~18 o F) at the inlet of each stage. Figure 6 shows the variation in degree of superheat at the inlet of second stage of compression with respect to intermediate pressure. Therefore degree of superheat needed theoretically are 33.5683°C, 21.6453°C, 15.5282°C and 9.3973°C respectively for given mass injection ratio, m r . Here the only value corresponding to the degree of superheat <11.11°C is for m r = 0.15. Thus the degree of superheat required for safer working of compressor limits the mass injection ratio to 0.15 or less than that.

Case 2: T cond = 60°C and T eva = 7.22°C
Now, the evaporator temperature is increased to 7.22°C keeping condenser temperature constant at 60°C. Interestingly, the experimental isentropic efficiency = 0.722 and isentropic efficiency obtained from best fit graph = 0.7225. The difference between the experimented isentropic efficiency and isentropic efficiency is negligible. Various values of COP of the cycle with respect to fifty values of intermediate pressure ratio ranging from 0.001 to 0.999   superheat, similar method can be used which is used to predict the performance for above 2 cases. The value of isentropic efficiency can be calculated by a MATLAB code from the polynomials given in Eq. 16 figure 10. However, the COP of refrigeration system of same capacity but using a single stage compressor is 1.7379. Cheng et al., analyzed non-azeotropic mixture R32/R1234ze (E) in a heat pump and predicted COP of ~3 for mass injection ratio of 0.15 for a vapor injection system [25]. Then for a refrigeration system the COP would be ~2 and our value is near to it. 0.05, 0.1, 0.125, 0.15 are 31.38°C, 19.96°C, 14.14°C and 8.33°C respectively as shown in Figure 9. The degree of superheat for mass injection ratio 0.15 is 8.33 which suggests that the maximum value of mass injection ratio should be less than 0.15 for the safer working of the compressor.

Case 3: T cond = 65°C and T eva = 1.66°C
Now the condenser temperature and evaporator temperatures are changed to 65°C and 1.66°C respectively (data for this condition is not available in Copeland webpage). To predict the COP, discharge temperature and degree of     The obtained value of degree of superheat is slightly higher than 11.11°C for mass injection ratio 0.15. The results shows that even in a very hot tropical climate when the condensing temperature becomes very high due to increase in the ambient temperature the two-phase injection system can helps prevent the compressor discharge temperature to not reach beyond 120 o C.

Case 4: T cond = 65°C and T eva = 7.22°C
The evaporator temperature is considered to be 7.22°C and condensing temperature as 65°C, 50 iterations were performed. Figure 13  The COP of refrigeration cycle using single stage compressor working within same temperature limit is 2.1304 which is even higher than the reported COP by Yang et al. [19]. The COP obtained at r p = 0.3 and said values of m r are 8.43%, 7.82%, 7.28% and 6.57% lower than the COP of refrigeration system of identical capacity but using a single stage compressor. Figure 14 shows In summary, the best mass injection ratio is 0.15 as per the simulation results of this work. Consolidated values of maximum COP, minimum degree of superheat and minimum discharge temperature for each cases are plotted together at mass injection ratio of 0.15 ( Figure 16). Following the trend of graph in Figure 16 it is assuring that increasing evaporator temperature and lowering condenser temperature increases COP [26]. Moreover, low condenser temperature is also related to lesser degree of superheat and discharge temperature. It is evident that the maximum discharge temperature is less than 120°C even for extreme operating temperature, say, 65°C condenser temperature and 1.66°C evaporator temperature ( figure  15). Thus the refrigerant R32 is safe at high atmospheric temperature. The degree of superheat at the inlet of second stage of compressor is also greater than 8°C which ensures no liquid refrigerant enters the scroll compressor. Xu et al. performed experiment on vapor injection system using R32 and R410a for various mass injection ratio, for extreme cooling at ambient temperature of 46°C [27]. However, condenser temperature will be ~15 to 20°C higher than the atmosphere temperature (in summer it can go up to 65°C) [28,29]. Xu et al. reported ~15 to 20% decrease in cooling COP of the system when employing vapor injection system [27]. The maximum decrease in COP observed for two phase injection system was ~9%. The minimum discharge temperature obtained for R32 using vapor injection system was ~122°C for vapor injection ratio of 0.4. In case of twophase injection system, the discharge temperature for mass injection ratio of 0.15 is ~110°C. Thus two-phase injection system is effective in decreasing the compressor discharge temperature. Shuxe et al. reported the experimental results for various ambient (condenser) temperature enhanced vapor injection system [30]. The enhanced vapor injection seems to increase the COP of the system depending on the vapor mass injection ratio. Between ambient temperatures 45°C and 40°C ~15% decrease in COP was observed. When compared to enhanced vapor injection system two phase injection system (proposed here) seems advantageous because the maximum decrease in COP for present system is ~9%. Guo et al. did experiment on R32 for air-conditioning and refrigeration application using the enhanced vapor injection approach [31]. The maximum decrease in discharge temperature reported is of 20-30°C. The maximum discharge temperature decrease obtained in this work  is ~40°C for 65°C condenser temperature (1.66°C evaporator temperature). Therefore, the use of two-phase injection process can decrease the discharge temperature of R32 and may keep the COP of the refrigeration system competitive [32]. The optimum value of mass injection ratio depends on the operating conditions. The minimum degree of superheat required at the inlet of each stage of the compressor also affects the maximum value of mass injection ratio.

CONCLUSIONS
The modelling and analysis of refrigeration systems with two-phase injection is carried out using a MATLAB code. In order to establish the model, the operating conditions has been taken from a commercial scroll compressor data sheet. Hot atmospheric temperature has been considered for the COPELAND TM Scroll compressor for analyzing R32 performance. It is evident from the results that two-phase injection system at mass injection 0.15 decreases the discharge temperature and also ensures that no liquid refrigerant enter the scroll compressor. When compared to a conventional single stage system, a two phase system would operate at low discharge temperature (~120 o C) without compromising the system coefficient of performance and energy efficiency ratio.

AUTHORSHIP CONTRIBUTIONS
Authors equally contributed to this work.

DATA AVAILABILITY STATEMENT
The authors confirm that the data that supports the findings of this study are available within the article. Raw data that support the finding of this study are available from the corresponding author, upon reasonable request.

CONFLICT OF INTEREST
The author declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

ETHICS
There are no ethical issues with the publication of this manuscript.