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Leakage analysis and concentration distribution of flammable refrigerant R290 in the automobile air conditioner system
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Leakage analysis and concentration distribution of flammable refrigerant R290 in the automobile air conditioner system Yun Zhang , Cichong Liu , Tianying Wang , Leyan Pan , Wanyong Li , Junye Shi , Jiangping Chen PII: DOI: Reference:
S0140-7007(19)30466-9 https://doi.org/10.1016/j.ijrefrig.2019.11.001 JIJR 4568
To appear in:
International Journal of Refrigeration
Received date: Revised date: Accepted date:
31 August 2019 16 October 2019 3 November 2019
Please cite this article as: Yun Zhang , Cichong Liu , Tianying Wang , Leyan Pan , Wanyong Li , Junye Shi , Jiangping Chen , Leakage analysis and concentration distribution of flammable refrigerant R290 in the automobile air conditioner system, International Journal of Refrigeration (2019), doi: https://doi.org/10.1016/j.ijrefrig.2019.11.001
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Highlights
The leak pressure, leak hole size and wind speed are the three major parameters that affect R290 distribution. Increasing the hole size and decreasing the wind speed will raise the maximum R290 concentration and the duration time. A lower pressure indicates that the concentration rises slower but a maintained longer time. Evaporator leakage is the most risky situation of the R290 MAC system. The highest concentration and the duration time can be both significantly reduced by the SL system.
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Leakage analysis and concentration distribution of flammable refrigerant R290 in the automobile air conditioner system a Yun Zhang , Cichong Liua, Tianying Wanga, Leyan Pana, Wanyong Lia, Junye Shia,b, Jiangping Chena,b*, a Institute of Refrigeration and Cryogenics, Shanghai Jiaotong University, Shanghai, China b Shanghai High Efficient Cooling System Research Center, Shanghai, China * Corresponding author at: Institute of Refrigeration and Cryogenics, Shanghai Jiaotong University, Shanghai, China. E-mail address: [email protected] (J. Chen). 021-34206775 Abstract The natural refrigerant R290 has been gradually investigated as an alternative refrigerant of R134a in the automobile air conditioning (MAC) systems at present. As an A3 class refrigerant, the application of R290 is associated with the greatest challenge of its flammable and explosive characteristics. This paper aimed to describe the distribution of R290 gas under different leakage conditions through experiments in the engine compartment and the passenger compartment. Our results showed that, evaporator leakage was the most risky situation of the system. In this case, R290 gas leaked to the passenger compartment, as a result, the R290 concentration in the environment was above the lower flammable limit (LFL). In the case of leakage in the engine compartment, the leak pressure, leak hole size, and wind speed were recognized as the three main parameters affecting R290 distribution. Moreover, increasing the hole size and decreasing the wind speed would raise the maximum R290 concentration as well as the duration in which the R290 gas concentration was above the LFL level. Additionally, a lower pressure suggested that the R290 concentration rose slower but it was maintained for a longer time above the LFL than that under high pressure situation. Keyword R290; automobile air conditioning; leakage; gas distribution Nomenclature GWP global warming potential LFL lower flammable limit ODP ozone depleting potential MAC automobile air conditioning HVAC heating ventilation and air conditioning Greek symbols R290 pressure in the pipe, Pa 𝑃1 atmospheric pressure, Pa 𝑃𝑎 𝑚̇ R290 mass flow rate, kg s −1 𝑢𝑉 R290 gas leaking speed, m s −1 𝑢𝑊 face velocity of the wind, m s−1 𝜌𝑎 density of the atmosphere, kg m−3 ; 𝜌𝑅290 density of the gas R290, kg m−3 ; diameter of the leakage hole, m ξ turbulent Schmidt number 𝜆 2
κ
adiabatic exponent cycle. An experimental study in a low temperature environment heat pump system showed that, the R290 system achieves a 6.5% higher COP comparing to the original R22 system. And the exhaust temperature is decreased to 80℃, which is 36% lower than the R22 system (Yun Zhang, 2019). Moreover, R290 is a by-product of petroleum and natural gas industry. It is cheap and compatible with the existing R22 system, which has good economy compared with other alternatives (Shouguang Y, 2002). Therefore, the natural refrigerant R290 has been widely used in household and commercial air conditioning (AC) systems (Xiaoning Chen, 2018). In MAC systems, R290 has the best heat pump system performance in cold climate compared with R134a and CO2 (Cichong Liu,2018). R290 can also be mixed with CO2 at a mass fraction of 70/30; therefore, COP can be improved to a maximum of 22% in the CO2 MAC systems (Binbin Yu,2018). Consequently, R290 is a promising alternative refrigerant of R134a in the MAC systems given these extraordinary performances (Sánchez D, 2017). As an A3 class refrigerant, the application of R290 is linked with the biggest challenge due to its flammable and explosive characteristics (Colbourne D, 2004). On this account, a series of safety precautions are needed to avoid leakage. Up to now, studies on the safety and leakage of R290 in the indoor ACs and heat pump systems have been carried out by many scholars. For instance, Colbourne D (2013) had established a theoretical model to describe the leakage situation of the commercial R290 ACs. Li, Tingxun
1. Introduction At present, the hydrofluorocarbon (HFC) refrigerant, such as R134a, has been widely used in the automobile air conditioning (MAC) systems, which is associated with a high global warming potential (GWP) value, thus intensifying the greenhouse effect worldwide (Mcculloch et al, 2003). In October 2016, the 28th Meeting of the Parties of the Montreal Protocol had reached a consensus to amend the Montreal Protocol to gradually phase down HFCs in 2019 (Hu L, 2017) in its Decision XXVIII/2. As a result, searching for the substitutions for HFCs has become an industrial and academic focus, and increasing attention has been paid to the impact of refrigerant emissions of the MAC systems on global warming. Currently, HFO-1234yf and CO2 are the two internationally recognized alternative refrigerants for the MAC systems (Vaghela, 2017). However, significant drawbacks, including the high cost of R1234yf (Devecio, 2017) and the excessive operating pressure of CO2 (Bai T, 2015), have hindered their widespread promotion and application globally. R290 (propane) is a type of environmentally-friendly refrigerant, which has the ozone destruction potential (ODP) value of 0 and the GWP value of <10, and these have conformed to the current environmental protection concept and meet the most stringent regulatory requirements. According to the theoretical analysis (C S Choudhari, 2017), refrigerant mass flow rate required with R290 is lower by 50 % compared to R22 at the similar system COP on a standard vapor compression 3
was applicable to the “light” gases (with the same or less density than air) and was extended to the “heavy” gases (with higher density than air) under a number of assumptions (Ooms et al., 1984).
(2014) had conducted a study regarding the household R290 ACs. In addition to the AC systems, risk assessment has also been carried out on the heat pump systems (Weier T, 2018). Compared with the indoor AC systems and heat pump systems, the MAC systems have more variable working conditions and environments. However, no study is conducted to examine R290 leakage in the MAC systems. Consequently, this paper aimed to describe the R290 gas distribution under different leakage conditions through experiments in the engine compartment and the passenger compartment. 2. Theoretical analysis of R290 leakage To ascertain R290 gas distribution after leakage, the gas ejection and diffusion models are required. Leakage of the MAC systems is usually induced by pipeline vibration or corrosion (Thomas L, 1981). Although the refrigerant in the MAC systems is usually in the two-phase state of gas-liquid, most refrigerant ejects in the manner of gas with a little liquid drop when leakage occurs. Typically, this process can be approximated as the adiabatic expansion of the ideal gas (Tang W, 2017). Therefore, apart from the thermophysical parameters of the refrigerant itself, the leak pressure 𝑃1 and leak hole size ξ are the two major parameters determining the gas leaking rate from the hole 𝑢𝑉 . To describe the gas diffusion process around the leakage position, Ooms (1972) had proposed a plume model (shown as Fig.1) to simulate the plume path of gases leaking into the atmosphere under atmospheric temperature and pressure. This model
Fig.1 Schematic diagram of the plume model. In this plume model, the gas concentration was cylindrically symmetric around the plume axis. In the plume, the concentration at an arbitrary point 𝑐(𝑠, 𝑟) was calculated by Eqs. (1): 𝑐(𝑠, 𝑟) = 𝑐 ∗ 𝑒𝑥𝑝(
−𝑟 2 ) 𝜆2 𝑏𝑠2
(1) Where 𝑐 represents the gas concentration on the plume axis (VOL); 𝑟 stands for the radial distance to the plume axis in a normal plume section (m); 𝜆2 (≈ 1.35) is the so-called turbulent Schmidt number; 𝑏𝑠 indicates the local characteristic width of the plume (m); In Eqs.(1), 𝑐 ∗ and 𝑏𝑠 were the unknown quantities; therefore, Khan and Abbasi (1999, 2000) had developed the empirical equations Eqs.(2) and Eqs.(3), and defined a dimensionless number 𝑄𝑓 Eqs.(4) to solve the parameters 𝑐 ∗ and ∗
4
𝑏𝑠
the R290 concentration in the same position is determined by Pa , P1 , and κ, ρ1 , ρa , ρR290 , uW , uV . Under specific environmental conditions, Pa and κ , ρ1 , ρa , ρR290 are the known quantities. Therefore, P1 , uV , and uW are the three major parameters affecting the R290 concentration. In our experiment, uV was calculated by Eqs. (5):
∗
𝑐 = (−0.0161965 + 0.00543481 ∗ 𝑄𝑓 − 0.00128631 ∗ 𝑄𝑓2 ) ∗ ln(𝑠) + 0.183894 ∗ 𝑒 −0.382484∗𝑄𝑓
(2) 𝑏𝑠 = (−0.0157719 + 0.104969 ∗ 𝑄𝑓 − 0.0347085 ∗ 𝑄𝑓2 ) ∗ 𝑙𝑛(𝑠) + 0.953469 ∗ 𝑄𝑓0.22422
(3) 𝑄𝑓 = (
𝑢𝑉 =
𝑢𝑊 𝜌𝑎 )( ) 𝑢𝑉 𝜌𝑅290
4𝑚̇ 𝜋 𝜌1 𝜉 2
(5) Where 𝑚̇ is the mass flow rate of R290 (kg s−1 ); ξ represents the equivalent diameter of the leakage hole (m); Based on the above analyses, the main factors that affect the distribution of R290 gas concentration after leakage include the leakage pressure P1 , leakage hole ξ and wind speed uW . Consequently, these three parameters would be discussed through various experiments.
(4) Where 𝑢𝑊 is the face velocity of wind (m s −1 ); 𝑢𝑉 represents the gas leaking rate from the leaking position (m s −1 ); 𝜌𝑎 indicates the atmosphere density (kg m−3 ); 𝜌𝑅290 suggests the density of the plume gas R290 (kg m−3 ); s represents the distance along the plume axis from the leaking position to a certain point (m); As figured out from Eqs. (1) to (4),
3. Experimental apparatuses and procedures Tab.1 Experimental Conditions No. The leakage position P1 /Mpa ξ/mm uW /(m s −1 ) 1 Condenser 2.5 2.0 0 2 Condenser 2.5 0.5 0 3 Condenser 1.5 2.0 0 4 Condenser 2.5 2.0 1 5 Condenser 2.5 2.0 2 6 Condenser 2.5 2.0 3 7 Compressor 2.5 0.5 0 8 Compressor 2.5 2.0 0 9 Evaporator 1.5 0.5 / 10 Evaporator 1.5 2.0 / 3.1 Test conditions the R290 distribution after leakage. To
According to the plume model, the leakage pressure P1 , the diameter of leakage hole ξ, and the wind speed uW are the three major parameters that affect
evaluate the impact of various factors, the following test conditions were selected after taking into consideration of the common test conditions of MAC, 5
as shown in Tab.1 As shown above, condenser (in front of the engine), compressor (behind the engine) and evaporator (in the passenger compartment) were the three leakage positions in this test. Specifically, the leakage position of condenser was near the outlet of the condenser in the MAC system, which was in the front of the engine compartment. Similarly, the leakage position of compressor was near the outlet of compressor in the MAC system, which was behind the engine. These two positions were inside the engine compartment, whereas that of evaporator was at the wind outlet of the HVAC in the passenger compartment. Three positions were displayed as leakage point A, B and C in Fig.2. Moreover, the wind speed of the blower in HVAC was 6m/s, while the leakage amount was set as 250 g according to the normal situation (Karthikeyan K, 2017).
different test conditions. The apparatuses used in the experiment are shown in Fig.2, including electronic scale, accumulator, gas ball valves, pressure transducer, electric heaters, vacuum pumps and pins of different sizes. The electronic scale was used to measure the leakage amount of R290. Pins of different diameters (shown in Fig.3) were utilized to control the leak hole size.
Fig.3 Pins to control the hole size. The accumulator was used to store the R290 gas, which could be heated by electric heaters to control the leakage pressure of R290. A pressure sensor was set in the pipeline to measure the leakage pressure. In addition, three on/off valves were set to switch the leakage position of R290. Besides, an electric fan was arranged in front of the vehicle to provide the face wind, thus setting the wind speed.
3.2 Experimental apparatus and set-up
To confirm the effects of these factors on R290 distribution, the following pipelines were designed to simulate the situation of R290 leakage under
6
Fig.2 Schematic diagram of the pipeline layout
In this experiment, 8 concentration sensors were set at the location of sensors were arranged, which were condenser (in front of the engine), and located in the engine compartment and compressor (behind the engine). the passenger compartment separately. Meanwhile, two more sensors were also Tab.3 displays the detail locations. arranged near the engine exhaust With reference to the SAE report pipeand on the battery, respectively, about R1234yf MAC leak risk which represented the high temperature assessment (Thomas A, 2013), 4 sensors surface and the potential ignition source. were set at 1 m above the floor at each The specific positions are exhibited in seat in the passenger compartment under Fig.4-6. Tab.2 displays the uncertainties the major consideration of passenger of the test equipment. smoking. In the engine compartment, Tab.2 Uncertainties of the test equipment Name
Uncertainty
Presser sensor (kPa)
±0.25%
R290 concentration sensor (%VOL)
±0.045% ±1
Electronic scale (g) Data logger Agilent)
(34927A,
7
0.004% dcV accuracy
Fig.4 Location of sensors in the passenger compartment
Fig.5 Location of sensors in the engine compartment
Fig.6 Location of sensors in the engine compartment 6 Tab.3 Location of different R290 sensors Compressor No. 1 2 3 4 5
7 Battery 8 Engine exhaust pipe 3.3 Experimental methods and procedures
Position Main driving position Assistant driving position Back seat on the right Back seat on the left Condenser
In this experiment, the R290 gas was initially stored in the accumulator, and the leakage pressure P1 in the pipeline 8
was adjusted through heating the accumulator. When the gas was heated to the specified pressure, the opening of valves 1, 2 and 3 would be corresponding to the leakage points of compressor (point B), condenser (point A) and evaporator (point C),
respectively. Then, pins of different sizes were used to control the leak hole size ξ, and the wind speed uW was set by the electric fan. The engine cover and the glass in the passenger compartment were closed during the test.
4. Experimental results and discussion Tab.4 Data regarding conditions 1 and 3 No. 1 1 1 3 3 3
Sensor location
Concentration of R290 (VOL)
5 7 8 5 7 8
6.99 2.23 3.66 4.53 2.21 2.26
This experiment mainly determined the effects of leakage pressure P1 , diameter of leakage hole ξ and wind speed uW on R290 gas distribution after leakage. Additionally, some safety design suggestions were also formulated for the R290 MAC system based on our experimental data.
Duration above the LFL level (s) 43 5 22 55 30 41
was rapidly ejected from the hole, and R290 concentration near the leak point rose rapidly, but it was also rapidly decreased after R290 injection in the pipeline was completed. In the case of low leakage pressure (1.5Mpa), which indicated that the vehicle was stop and the MAC system was not working, R290 would spread out evenly from the hole, the R290 concentration in the engine compartment rises at a slower rate, but it maintained for a longer duration than that under high pressure situation due to a longer leak time.
4.1 Effect of leak pressure P1
To determine the influence of P1 , data on the test conditions 1 and 3 were compared. The results are shown in Tab.4. Apparently, the highest concentration points of different P1 were both at the leak point in condenser (location 5), which had the longest duration above the LFL. For detailed analysis, curves regarding these two concentrations were plotted, as displayed in Fig.7 (the red line indicates the LFL of R290, 2.1%). It could be concluded that, a higher P1 resulted in a higher R290 concentration. However, when the leakage pressure was high (2.5Mpa), which suggested system leakage during operation, R290
Fig.7 Concentration curves of different leak pressures 9
Tab.5 Data on conditions 1 and 2 No. 1 1 1 2 2 2
Sensor location
Duration above the LFL level (s) 43 5 22 0 0 0 4.3 Influence of wind speed uW
Concentration of R290 (VOL)
5 7 8 5 7 8
6.99 2.23 3.66 0.66 0.56 0.57
4.2 Influence of leak hole size ξ
To determine the influence of ξ, data on the test conditions 1 and 2 were compared. The results are shown in Tab.5. When ξ was 2 mm, the highest concentration of R290 at different locations were all above the LFL level. When ξ was 0.5 mm, the R290 concentration was still lower than the LFL level. For detailed analysis, two concentration curves at location 5 were plotted together in Fig.8 (the red line stands for the LFL of R290, 2.1%). The results showed that, the maximum R290 concentration was lower than 0.6% when the leak hole size ξ was 0.5 mm, which was mainly because that R290 leaked very slowly in the case of small leak hole, and the R290 gas diffused at the mean time, suggesting that the concentration accumulation could hardly been formed.
To determine the influence of uW , the control groups 1,4,5 and 6 with the same hole size ξ of 2 mm, same leak pressure P1 of 2.5 Mpa were chosen for detailed analysis. The results are displayed in Tab.6. As shown above, the highest concentration points were all at the leak point in the condenser. The concentration curves were plotted together, as exhibited in Fig.9 (the red line indicates the LFL of R290, 2.1%).
Fig.9 Concentration curves of different wind speeds It could be concluded from Fig.9 that, when the R290 gas leaked into the engine compartment, the concentration distribution was greatly affected by the wind speed uW . With the increase in uW , the maximum R290 concentration and the duration above the LFL level were gradually decreased. From this point of view, the R290 MAC system had a lower risk of leakage when the
Fig.8 Concentration curves of different leak hole sizes 10
vehicle was moving. Tab.6 Data on conditions 1,4,5, and 6 No.
Sensor location
Concentration of R290 (VOL)
1 5 1 7 1 8 4 5 4 7 4 8 5 5 5 7 5 8 6 5 6 7 6 8 4.4 Influence of the leak position
6.99 2.23 3.66 6.75 1.46 3.49 5.06 1.28 3.41 4.57 0.97 2.26
Duration above the LFL level (s) 43 5 22 47 0 27 31 0 18 24 0 10
shown in Tab.7. Under these two According to the leakage position, conditions, the maximum R290 gas leakage of the R290 MAC system is concentration of the four sensors were divided into two types, including engine all above the LFL level, and the duration compartment leakage and passenger was over 130 s. Unlike the leakage in compartment leakage. Specifically, the engine compartment, the R290 gas leakage of condenser and compressor accumulated in the passenger occurs in the engine compartment, while compartment after the leakage. leakage of evaporator takes place in the Therefore, smaller leak hole size means passenger compartment. more duration time due to a longer leak The results of conditions 9, 10 are time. Tab.7 Data on conditions 9 and 10 No. Sensor location Concentration of R290 (VOL) Duration above the LFL level (s) 9 1 4.14 238 9 2 3.09 212 9 3 2.98 274 9 4 3.26 162 10 1 5.88 185 10 2 4.52 174 10 3 4.22 151 10 4 4.76 132
For detailed analysis, the concentration curve of R290 gas in the passenger compartment of test condition 10 (leak pressure P1 =1.5 Mpa, and leak hole size ξ=2 mm) were selected, as displayed in Fig.10. Clearly, the highest
R290 concentration in the vehicle was 5.88 VOL at the main driving position, which was maintained for over 130 s above the LFL level.
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gas in the engine compartment in most cases, leading to a lower risk of combustion.
Fig.10 Concentration curve of condition 10 It was concluded according to the data obtained from test conditions 9 and 10 that, R290 gas in the pipeline was sent to the passenger compartment by the blower in the HVAC in the case of leakage in the passenger compartment, which caused high concentration of R290 gas accumulated in the passenger compartment for a long time. At this moment, the risk of combustion in the passenger compartment was high. According to the data obtained from the test conditions 1-8, R290 concentration in the engine compartment was greatly affected by ξ and uW when the engine compartment leakage occurred, and there was no long-time accumulation of high concentration of R290 gas. Therefore, the risk of combustion in the engine compartment was much lower.
Fig.11 Leakage risk analysis of the R290 MAC system On this basis, some effective safety measures can be taken to reduce the potential safety risk of the R290 MAC system. To avoid leakage in the passenger compartment, a secondary loop (SL) MAC system (Li G et al, 2014) can be adopted. Plate heat exchangers can be used to replace the conventional evaporator and condenser, so that the heat can be transported by the non-flammable fluid from the engine compartment to the passenger compartment, which suggests that the flammable refrigerant can be physically isolated from the passenger compartment. Additionally, the R290 concentration sensors can be installed at the air outlet of HVAC. When R290 leakage is detected in the passenger compartment, the solenoid valve should be cut off immediately to ensure that only a small amount of R290 will leak into the compartment. When R290 leakage is detected in the engine compartment, the front fan should be speeded up to increase the upwind volume for diluting
4.5 Safety suggestions for the R290 MAC
The general risk level of the R290 MAC system was described, as presented in Fig 11. In the case of evaporator leakage, the concentration of R290 gas in the compartment would exceed the LFL line for a long time, leading to an increased risk of combustion. In the case of condenser or compressor leakage, there would be no sustained high concentration of R290 12
the R290 gas, so that the concentration of R290 gas will be decreased rapidly.
the SL system have been experimentally compared. The test pipeline of the SL system (shown in Fig.12) was similar to the original DX system. Two plate heat exchangers were used to replace the evaporator and condenser.
4.6 Safety improvement effect of the SL system
In this paper, the differences of R290 gas distribution after leakage between the direct expansion (DX) system and
Fig.12 Schematic diagram of the SL system The whole AC system (without the compressor) was installed in a box (shown in Fig.13), so that the heat can be transported by the non-flammable fluid from the engine compartment to the passenger compartment and the flammable refrigerant can be physically isolated from the passenger compartment.
Fig.13 Diagram of the SL box In order to validate the safety improvement effect of the SL system. Conditions 11-14 (shown in Tab.8) were set to compare with conditions 1,4,5,6. Same hole size ξ of 2 mm, same leak pressure P1 of 2.5 Mpa were chosen for 13
detailed analysis. Tab.8 Experimental Conditions No. 11 12 13 14
The leakage position SL box SL box SL box SL box
P1 / Mpa
ξ/ mm
uW / (m s −1 )
2.5 2.5 2.5 2.5
2.0 2.0 2.0 2.0
0 1 2 3
Fig.14 Concentration curves of different wind speeds in the SL system 5. Conclusions Based on the theoretical analyses on leakage of the R290 MAC system, the leak pressure P1 , leak hole size ξ, and wind speed uW are the three major parameters that affect R290 distribution. This paper has conducted various experiments on these three parameters, and some safety design suggestions are formulated for the R290 MAC system on the basis of our experimental data: (1) Evaporator leakage is the most risky situation of the R290 MAC system. In this case, R290 gas leaks to a confined compartment, as a result, there is a high R290 gas concentration in the environment, which is maintained at above the LFL level. (2)The leak pressure P1 , leak hole size ξ, and wind speed uW are the three major parameters that affect the R290 concentration after leakage. Increasing the hole size and decreasing the wind speed will raise the maximum R290 concentration and the duration of the gas concentration. As for the leak pressure, a lower pressure indicates that the R290 concentration rises slower but maintains for a longer time above the LFL level than that under a high pressure situation. (3)Mitigation measures, includingSL system or installation of the R290 concentration sensors, can be adopted to
The results are displayed in Tab.9. Similar to the DX system, the highest concentration point of the SL system was also at the leak point. Moreover, sensor readouts of 1-4 (in the passenger compartment) remained 0 during the SL leakage, which indicated the SL system can effectively isolate the R290 gas from the passenger compartment. Tab.9 Data on conditions 1,4,5,6, 11-14 No 1 4 5 6 11 12 13 14
Concentration of R290 (VOL) 6.99 6.75 5.06 4.57 3.71 3.48 3.03 2.53
Duration above the LFL level (s) 43 47 31 24 38 28 26 18
The concentration curves were plotted together, as exhibited in Fig.14 (the red line indicates the LFL of R290, 2.1%). Compared with Fig.9, it can be clearly to find out that under different wind speed conditions the highest concentration of R290 and the duration time above the LFL level of the SL system decreased significantly compared to the DX system.
14
reduce the combustion risk of the R290 MAC system. The highest concentration of R290 and the duration time above the
LFL level can be both significantly reduced by the SL system.
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Declaration of interests
☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
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Leakage analysis and concentration distribution of flammable refrigerant R290 in the automobile air conditioner system Yun Zhang , Cichong Liu , Tianying Wang , Leyan Pan , Wanyong Li , Junye Shi , Jiangping Chen PII: DOI: Reference:
S0140-7007(19)30466-9 https://doi.org/10.1016/j.ijrefrig.2019.11.001 JIJR 4568
To appear in:
International Journal of Refrigeration
Received date: Revised date: Accepted date:
31 August 2019 16 October 2019 3 November 2019
Please cite this article as: Yun Zhang , Cichong Liu , Tianying Wang , Leyan Pan , Wanyong Li , Junye Shi , Jiangping Chen , Leakage analysis and concentration distribution of flammable refrigerant R290 in the automobile air conditioner system, International Journal of Refrigeration (2019), doi: https://doi.org/10.1016/j.ijrefrig.2019.11.001
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Highlights
The leak pressure, leak hole size and wind speed are the three major parameters that affect R290 distribution. Increasing the hole size and decreasing the wind speed will raise the maximum R290 concentration and the duration time. A lower pressure indicates that the concentration rises slower but a maintained longer time. Evaporator leakage is the most risky situation of the R290 MAC system. The highest concentration and the duration time can be both significantly reduced by the SL system.
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Leakage analysis and concentration distribution of flammable refrigerant R290 in the automobile air conditioner system a Yun Zhang , Cichong Liua, Tianying Wanga, Leyan Pana, Wanyong Lia, Junye Shia,b, Jiangping Chena,b*, a Institute of Refrigeration and Cryogenics, Shanghai Jiaotong University, Shanghai, China b Shanghai High Efficient Cooling System Research Center, Shanghai, China * Corresponding author at: Institute of Refrigeration and Cryogenics, Shanghai Jiaotong University, Shanghai, China. E-mail address: [email protected] (J. Chen). 021-34206775 Abstract The natural refrigerant R290 has been gradually investigated as an alternative refrigerant of R134a in the automobile air conditioning (MAC) systems at present. As an A3 class refrigerant, the application of R290 is associated with the greatest challenge of its flammable and explosive characteristics. This paper aimed to describe the distribution of R290 gas under different leakage conditions through experiments in the engine compartment and the passenger compartment. Our results showed that, evaporator leakage was the most risky situation of the system. In this case, R290 gas leaked to the passenger compartment, as a result, the R290 concentration in the environment was above the lower flammable limit (LFL). In the case of leakage in the engine compartment, the leak pressure, leak hole size, and wind speed were recognized as the three main parameters affecting R290 distribution. Moreover, increasing the hole size and decreasing the wind speed would raise the maximum R290 concentration as well as the duration in which the R290 gas concentration was above the LFL level. Additionally, a lower pressure suggested that the R290 concentration rose slower but it was maintained for a longer time above the LFL than that under high pressure situation. Keyword R290; automobile air conditioning; leakage; gas distribution Nomenclature GWP global warming potential LFL lower flammable limit ODP ozone depleting potential MAC automobile air conditioning HVAC heating ventilation and air conditioning Greek symbols R290 pressure in the pipe, Pa 𝑃1 atmospheric pressure, Pa 𝑃𝑎 𝑚̇ R290 mass flow rate, kg s −1 𝑢𝑉 R290 gas leaking speed, m s −1 𝑢𝑊 face velocity of the wind, m s−1 𝜌𝑎 density of the atmosphere, kg m−3 ; 𝜌𝑅290 density of the gas R290, kg m−3 ; diameter of the leakage hole, m ξ turbulent Schmidt number 𝜆 2
κ
adiabatic exponent cycle. An experimental study in a low temperature environment heat pump system showed that, the R290 system achieves a 6.5% higher COP comparing to the original R22 system. And the exhaust temperature is decreased to 80℃, which is 36% lower than the R22 system (Yun Zhang, 2019). Moreover, R290 is a by-product of petroleum and natural gas industry. It is cheap and compatible with the existing R22 system, which has good economy compared with other alternatives (Shouguang Y, 2002). Therefore, the natural refrigerant R290 has been widely used in household and commercial air conditioning (AC) systems (Xiaoning Chen, 2018). In MAC systems, R290 has the best heat pump system performance in cold climate compared with R134a and CO2 (Cichong Liu,2018). R290 can also be mixed with CO2 at a mass fraction of 70/30; therefore, COP can be improved to a maximum of 22% in the CO2 MAC systems (Binbin Yu,2018). Consequently, R290 is a promising alternative refrigerant of R134a in the MAC systems given these extraordinary performances (Sánchez D, 2017). As an A3 class refrigerant, the application of R290 is linked with the biggest challenge due to its flammable and explosive characteristics (Colbourne D, 2004). On this account, a series of safety precautions are needed to avoid leakage. Up to now, studies on the safety and leakage of R290 in the indoor ACs and heat pump systems have been carried out by many scholars. For instance, Colbourne D (2013) had established a theoretical model to describe the leakage situation of the commercial R290 ACs. Li, Tingxun
1. Introduction At present, the hydrofluorocarbon (HFC) refrigerant, such as R134a, has been widely used in the automobile air conditioning (MAC) systems, which is associated with a high global warming potential (GWP) value, thus intensifying the greenhouse effect worldwide (Mcculloch et al, 2003). In October 2016, the 28th Meeting of the Parties of the Montreal Protocol had reached a consensus to amend the Montreal Protocol to gradually phase down HFCs in 2019 (Hu L, 2017) in its Decision XXVIII/2. As a result, searching for the substitutions for HFCs has become an industrial and academic focus, and increasing attention has been paid to the impact of refrigerant emissions of the MAC systems on global warming. Currently, HFO-1234yf and CO2 are the two internationally recognized alternative refrigerants for the MAC systems (Vaghela, 2017). However, significant drawbacks, including the high cost of R1234yf (Devecio, 2017) and the excessive operating pressure of CO2 (Bai T, 2015), have hindered their widespread promotion and application globally. R290 (propane) is a type of environmentally-friendly refrigerant, which has the ozone destruction potential (ODP) value of 0 and the GWP value of <10, and these have conformed to the current environmental protection concept and meet the most stringent regulatory requirements. According to the theoretical analysis (C S Choudhari, 2017), refrigerant mass flow rate required with R290 is lower by 50 % compared to R22 at the similar system COP on a standard vapor compression 3
was applicable to the “light” gases (with the same or less density than air) and was extended to the “heavy” gases (with higher density than air) under a number of assumptions (Ooms et al., 1984).
(2014) had conducted a study regarding the household R290 ACs. In addition to the AC systems, risk assessment has also been carried out on the heat pump systems (Weier T, 2018). Compared with the indoor AC systems and heat pump systems, the MAC systems have more variable working conditions and environments. However, no study is conducted to examine R290 leakage in the MAC systems. Consequently, this paper aimed to describe the R290 gas distribution under different leakage conditions through experiments in the engine compartment and the passenger compartment. 2. Theoretical analysis of R290 leakage To ascertain R290 gas distribution after leakage, the gas ejection and diffusion models are required. Leakage of the MAC systems is usually induced by pipeline vibration or corrosion (Thomas L, 1981). Although the refrigerant in the MAC systems is usually in the two-phase state of gas-liquid, most refrigerant ejects in the manner of gas with a little liquid drop when leakage occurs. Typically, this process can be approximated as the adiabatic expansion of the ideal gas (Tang W, 2017). Therefore, apart from the thermophysical parameters of the refrigerant itself, the leak pressure 𝑃1 and leak hole size ξ are the two major parameters determining the gas leaking rate from the hole 𝑢𝑉 . To describe the gas diffusion process around the leakage position, Ooms (1972) had proposed a plume model (shown as Fig.1) to simulate the plume path of gases leaking into the atmosphere under atmospheric temperature and pressure. This model
Fig.1 Schematic diagram of the plume model. In this plume model, the gas concentration was cylindrically symmetric around the plume axis. In the plume, the concentration at an arbitrary point 𝑐(𝑠, 𝑟) was calculated by Eqs. (1): 𝑐(𝑠, 𝑟) = 𝑐 ∗ 𝑒𝑥𝑝(
−𝑟 2 ) 𝜆2 𝑏𝑠2
(1) Where 𝑐 represents the gas concentration on the plume axis (VOL); 𝑟 stands for the radial distance to the plume axis in a normal plume section (m); 𝜆2 (≈ 1.35) is the so-called turbulent Schmidt number; 𝑏𝑠 indicates the local characteristic width of the plume (m); In Eqs.(1), 𝑐 ∗ and 𝑏𝑠 were the unknown quantities; therefore, Khan and Abbasi (1999, 2000) had developed the empirical equations Eqs.(2) and Eqs.(3), and defined a dimensionless number 𝑄𝑓 Eqs.(4) to solve the parameters 𝑐 ∗ and ∗
4
𝑏𝑠
the R290 concentration in the same position is determined by Pa , P1 , and κ, ρ1 , ρa , ρR290 , uW , uV . Under specific environmental conditions, Pa and κ , ρ1 , ρa , ρR290 are the known quantities. Therefore, P1 , uV , and uW are the three major parameters affecting the R290 concentration. In our experiment, uV was calculated by Eqs. (5):
∗
𝑐 = (−0.0161965 + 0.00543481 ∗ 𝑄𝑓 − 0.00128631 ∗ 𝑄𝑓2 ) ∗ ln(𝑠) + 0.183894 ∗ 𝑒 −0.382484∗𝑄𝑓
(2) 𝑏𝑠 = (−0.0157719 + 0.104969 ∗ 𝑄𝑓 − 0.0347085 ∗ 𝑄𝑓2 ) ∗ 𝑙𝑛(𝑠) + 0.953469 ∗ 𝑄𝑓0.22422
(3) 𝑄𝑓 = (
𝑢𝑉 =
𝑢𝑊 𝜌𝑎 )( ) 𝑢𝑉 𝜌𝑅290
4𝑚̇ 𝜋 𝜌1 𝜉 2
(5) Where 𝑚̇ is the mass flow rate of R290 (kg s−1 ); ξ represents the equivalent diameter of the leakage hole (m); Based on the above analyses, the main factors that affect the distribution of R290 gas concentration after leakage include the leakage pressure P1 , leakage hole ξ and wind speed uW . Consequently, these three parameters would be discussed through various experiments.
(4) Where 𝑢𝑊 is the face velocity of wind (m s −1 ); 𝑢𝑉 represents the gas leaking rate from the leaking position (m s −1 ); 𝜌𝑎 indicates the atmosphere density (kg m−3 ); 𝜌𝑅290 suggests the density of the plume gas R290 (kg m−3 ); s represents the distance along the plume axis from the leaking position to a certain point (m); As figured out from Eqs. (1) to (4),
3. Experimental apparatuses and procedures Tab.1 Experimental Conditions No. The leakage position P1 /Mpa ξ/mm uW /(m s −1 ) 1 Condenser 2.5 2.0 0 2 Condenser 2.5 0.5 0 3 Condenser 1.5 2.0 0 4 Condenser 2.5 2.0 1 5 Condenser 2.5 2.0 2 6 Condenser 2.5 2.0 3 7 Compressor 2.5 0.5 0 8 Compressor 2.5 2.0 0 9 Evaporator 1.5 0.5 / 10 Evaporator 1.5 2.0 / 3.1 Test conditions the R290 distribution after leakage. To
According to the plume model, the leakage pressure P1 , the diameter of leakage hole ξ, and the wind speed uW are the three major parameters that affect
evaluate the impact of various factors, the following test conditions were selected after taking into consideration of the common test conditions of MAC, 5
as shown in Tab.1 As shown above, condenser (in front of the engine), compressor (behind the engine) and evaporator (in the passenger compartment) were the three leakage positions in this test. Specifically, the leakage position of condenser was near the outlet of the condenser in the MAC system, which was in the front of the engine compartment. Similarly, the leakage position of compressor was near the outlet of compressor in the MAC system, which was behind the engine. These two positions were inside the engine compartment, whereas that of evaporator was at the wind outlet of the HVAC in the passenger compartment. Three positions were displayed as leakage point A, B and C in Fig.2. Moreover, the wind speed of the blower in HVAC was 6m/s, while the leakage amount was set as 250 g according to the normal situation (Karthikeyan K, 2017).
different test conditions. The apparatuses used in the experiment are shown in Fig.2, including electronic scale, accumulator, gas ball valves, pressure transducer, electric heaters, vacuum pumps and pins of different sizes. The electronic scale was used to measure the leakage amount of R290. Pins of different diameters (shown in Fig.3) were utilized to control the leak hole size.
Fig.3 Pins to control the hole size. The accumulator was used to store the R290 gas, which could be heated by electric heaters to control the leakage pressure of R290. A pressure sensor was set in the pipeline to measure the leakage pressure. In addition, three on/off valves were set to switch the leakage position of R290. Besides, an electric fan was arranged in front of the vehicle to provide the face wind, thus setting the wind speed.
3.2 Experimental apparatus and set-up
To confirm the effects of these factors on R290 distribution, the following pipelines were designed to simulate the situation of R290 leakage under
6
Fig.2 Schematic diagram of the pipeline layout
In this experiment, 8 concentration sensors were set at the location of sensors were arranged, which were condenser (in front of the engine), and located in the engine compartment and compressor (behind the engine). the passenger compartment separately. Meanwhile, two more sensors were also Tab.3 displays the detail locations. arranged near the engine exhaust With reference to the SAE report pipeand on the battery, respectively, about R1234yf MAC leak risk which represented the high temperature assessment (Thomas A, 2013), 4 sensors surface and the potential ignition source. were set at 1 m above the floor at each The specific positions are exhibited in seat in the passenger compartment under Fig.4-6. Tab.2 displays the uncertainties the major consideration of passenger of the test equipment. smoking. In the engine compartment, Tab.2 Uncertainties of the test equipment Name
Uncertainty
Presser sensor (kPa)
±0.25%
R290 concentration sensor (%VOL)
±0.045% ±1
Electronic scale (g) Data logger Agilent)
(34927A,
7
0.004% dcV accuracy
Fig.4 Location of sensors in the passenger compartment
Fig.5 Location of sensors in the engine compartment
Fig.6 Location of sensors in the engine compartment 6 Tab.3 Location of different R290 sensors Compressor No. 1 2 3 4 5
7 Battery 8 Engine exhaust pipe 3.3 Experimental methods and procedures
Position Main driving position Assistant driving position Back seat on the right Back seat on the left Condenser
In this experiment, the R290 gas was initially stored in the accumulator, and the leakage pressure P1 in the pipeline 8
was adjusted through heating the accumulator. When the gas was heated to the specified pressure, the opening of valves 1, 2 and 3 would be corresponding to the leakage points of compressor (point B), condenser (point A) and evaporator (point C),
respectively. Then, pins of different sizes were used to control the leak hole size ξ, and the wind speed uW was set by the electric fan. The engine cover and the glass in the passenger compartment were closed during the test.
4. Experimental results and discussion Tab.4 Data regarding conditions 1 and 3 No. 1 1 1 3 3 3
Sensor location
Concentration of R290 (VOL)
5 7 8 5 7 8
6.99 2.23 3.66 4.53 2.21 2.26
This experiment mainly determined the effects of leakage pressure P1 , diameter of leakage hole ξ and wind speed uW on R290 gas distribution after leakage. Additionally, some safety design suggestions were also formulated for the R290 MAC system based on our experimental data.
Duration above the LFL level (s) 43 5 22 55 30 41
was rapidly ejected from the hole, and R290 concentration near the leak point rose rapidly, but it was also rapidly decreased after R290 injection in the pipeline was completed. In the case of low leakage pressure (1.5Mpa), which indicated that the vehicle was stop and the MAC system was not working, R290 would spread out evenly from the hole, the R290 concentration in the engine compartment rises at a slower rate, but it maintained for a longer duration than that under high pressure situation due to a longer leak time.
4.1 Effect of leak pressure P1
To determine the influence of P1 , data on the test conditions 1 and 3 were compared. The results are shown in Tab.4. Apparently, the highest concentration points of different P1 were both at the leak point in condenser (location 5), which had the longest duration above the LFL. For detailed analysis, curves regarding these two concentrations were plotted, as displayed in Fig.7 (the red line indicates the LFL of R290, 2.1%). It could be concluded that, a higher P1 resulted in a higher R290 concentration. However, when the leakage pressure was high (2.5Mpa), which suggested system leakage during operation, R290
Fig.7 Concentration curves of different leak pressures 9
Tab.5 Data on conditions 1 and 2 No. 1 1 1 2 2 2
Sensor location
Duration above the LFL level (s) 43 5 22 0 0 0 4.3 Influence of wind speed uW
Concentration of R290 (VOL)
5 7 8 5 7 8
6.99 2.23 3.66 0.66 0.56 0.57
4.2 Influence of leak hole size ξ
To determine the influence of ξ, data on the test conditions 1 and 2 were compared. The results are shown in Tab.5. When ξ was 2 mm, the highest concentration of R290 at different locations were all above the LFL level. When ξ was 0.5 mm, the R290 concentration was still lower than the LFL level. For detailed analysis, two concentration curves at location 5 were plotted together in Fig.8 (the red line stands for the LFL of R290, 2.1%). The results showed that, the maximum R290 concentration was lower than 0.6% when the leak hole size ξ was 0.5 mm, which was mainly because that R290 leaked very slowly in the case of small leak hole, and the R290 gas diffused at the mean time, suggesting that the concentration accumulation could hardly been formed.
To determine the influence of uW , the control groups 1,4,5 and 6 with the same hole size ξ of 2 mm, same leak pressure P1 of 2.5 Mpa were chosen for detailed analysis. The results are displayed in Tab.6. As shown above, the highest concentration points were all at the leak point in the condenser. The concentration curves were plotted together, as exhibited in Fig.9 (the red line indicates the LFL of R290, 2.1%).
Fig.9 Concentration curves of different wind speeds It could be concluded from Fig.9 that, when the R290 gas leaked into the engine compartment, the concentration distribution was greatly affected by the wind speed uW . With the increase in uW , the maximum R290 concentration and the duration above the LFL level were gradually decreased. From this point of view, the R290 MAC system had a lower risk of leakage when the
Fig.8 Concentration curves of different leak hole sizes 10
vehicle was moving. Tab.6 Data on conditions 1,4,5, and 6 No.
Sensor location
Concentration of R290 (VOL)
1 5 1 7 1 8 4 5 4 7 4 8 5 5 5 7 5 8 6 5 6 7 6 8 4.4 Influence of the leak position
6.99 2.23 3.66 6.75 1.46 3.49 5.06 1.28 3.41 4.57 0.97 2.26
Duration above the LFL level (s) 43 5 22 47 0 27 31 0 18 24 0 10
shown in Tab.7. Under these two According to the leakage position, conditions, the maximum R290 gas leakage of the R290 MAC system is concentration of the four sensors were divided into two types, including engine all above the LFL level, and the duration compartment leakage and passenger was over 130 s. Unlike the leakage in compartment leakage. Specifically, the engine compartment, the R290 gas leakage of condenser and compressor accumulated in the passenger occurs in the engine compartment, while compartment after the leakage. leakage of evaporator takes place in the Therefore, smaller leak hole size means passenger compartment. more duration time due to a longer leak The results of conditions 9, 10 are time. Tab.7 Data on conditions 9 and 10 No. Sensor location Concentration of R290 (VOL) Duration above the LFL level (s) 9 1 4.14 238 9 2 3.09 212 9 3 2.98 274 9 4 3.26 162 10 1 5.88 185 10 2 4.52 174 10 3 4.22 151 10 4 4.76 132
For detailed analysis, the concentration curve of R290 gas in the passenger compartment of test condition 10 (leak pressure P1 =1.5 Mpa, and leak hole size ξ=2 mm) were selected, as displayed in Fig.10. Clearly, the highest
R290 concentration in the vehicle was 5.88 VOL at the main driving position, which was maintained for over 130 s above the LFL level.
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gas in the engine compartment in most cases, leading to a lower risk of combustion.
Fig.10 Concentration curve of condition 10 It was concluded according to the data obtained from test conditions 9 and 10 that, R290 gas in the pipeline was sent to the passenger compartment by the blower in the HVAC in the case of leakage in the passenger compartment, which caused high concentration of R290 gas accumulated in the passenger compartment for a long time. At this moment, the risk of combustion in the passenger compartment was high. According to the data obtained from the test conditions 1-8, R290 concentration in the engine compartment was greatly affected by ξ and uW when the engine compartment leakage occurred, and there was no long-time accumulation of high concentration of R290 gas. Therefore, the risk of combustion in the engine compartment was much lower.
Fig.11 Leakage risk analysis of the R290 MAC system On this basis, some effective safety measures can be taken to reduce the potential safety risk of the R290 MAC system. To avoid leakage in the passenger compartment, a secondary loop (SL) MAC system (Li G et al, 2014) can be adopted. Plate heat exchangers can be used to replace the conventional evaporator and condenser, so that the heat can be transported by the non-flammable fluid from the engine compartment to the passenger compartment, which suggests that the flammable refrigerant can be physically isolated from the passenger compartment. Additionally, the R290 concentration sensors can be installed at the air outlet of HVAC. When R290 leakage is detected in the passenger compartment, the solenoid valve should be cut off immediately to ensure that only a small amount of R290 will leak into the compartment. When R290 leakage is detected in the engine compartment, the front fan should be speeded up to increase the upwind volume for diluting
4.5 Safety suggestions for the R290 MAC
The general risk level of the R290 MAC system was described, as presented in Fig 11. In the case of evaporator leakage, the concentration of R290 gas in the compartment would exceed the LFL line for a long time, leading to an increased risk of combustion. In the case of condenser or compressor leakage, there would be no sustained high concentration of R290 12
the R290 gas, so that the concentration of R290 gas will be decreased rapidly.
the SL system have been experimentally compared. The test pipeline of the SL system (shown in Fig.12) was similar to the original DX system. Two plate heat exchangers were used to replace the evaporator and condenser.
4.6 Safety improvement effect of the SL system
In this paper, the differences of R290 gas distribution after leakage between the direct expansion (DX) system and
Fig.12 Schematic diagram of the SL system The whole AC system (without the compressor) was installed in a box (shown in Fig.13), so that the heat can be transported by the non-flammable fluid from the engine compartment to the passenger compartment and the flammable refrigerant can be physically isolated from the passenger compartment.
Fig.13 Diagram of the SL box In order to validate the safety improvement effect of the SL system. Conditions 11-14 (shown in Tab.8) were set to compare with conditions 1,4,5,6. Same hole size ξ of 2 mm, same leak pressure P1 of 2.5 Mpa were chosen for 13
detailed analysis. Tab.8 Experimental Conditions No. 11 12 13 14
The leakage position SL box SL box SL box SL box
P1 / Mpa
ξ/ mm
uW / (m s −1 )
2.5 2.5 2.5 2.5
2.0 2.0 2.0 2.0
0 1 2 3
Fig.14 Concentration curves of different wind speeds in the SL system 5. Conclusions Based on the theoretical analyses on leakage of the R290 MAC system, the leak pressure P1 , leak hole size ξ, and wind speed uW are the three major parameters that affect R290 distribution. This paper has conducted various experiments on these three parameters, and some safety design suggestions are formulated for the R290 MAC system on the basis of our experimental data: (1) Evaporator leakage is the most risky situation of the R290 MAC system. In this case, R290 gas leaks to a confined compartment, as a result, there is a high R290 gas concentration in the environment, which is maintained at above the LFL level. (2)The leak pressure P1 , leak hole size ξ, and wind speed uW are the three major parameters that affect the R290 concentration after leakage. Increasing the hole size and decreasing the wind speed will raise the maximum R290 concentration and the duration of the gas concentration. As for the leak pressure, a lower pressure indicates that the R290 concentration rises slower but maintains for a longer time above the LFL level than that under a high pressure situation. (3)Mitigation measures, includingSL system or installation of the R290 concentration sensors, can be adopted to
The results are displayed in Tab.9. Similar to the DX system, the highest concentration point of the SL system was also at the leak point. Moreover, sensor readouts of 1-4 (in the passenger compartment) remained 0 during the SL leakage, which indicated the SL system can effectively isolate the R290 gas from the passenger compartment. Tab.9 Data on conditions 1,4,5,6, 11-14 No 1 4 5 6 11 12 13 14
Concentration of R290 (VOL) 6.99 6.75 5.06 4.57 3.71 3.48 3.03 2.53
Duration above the LFL level (s) 43 47 31 24 38 28 26 18
The concentration curves were plotted together, as exhibited in Fig.14 (the red line indicates the LFL of R290, 2.1%). Compared with Fig.9, it can be clearly to find out that under different wind speed conditions the highest concentration of R290 and the duration time above the LFL level of the SL system decreased significantly compared to the DX system.
14
reduce the combustion risk of the R290 MAC system. The highest concentration of R290 and the duration time above the
LFL level can be both significantly reduced by the SL system.
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Declaration of interests
☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
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