Application analysis of Wankel expander in CO2 trans-critical refrigeration cycle

doi:10.11949/0438-1157.20201570

Abstract

Abstract:

Utilizing expander instead of throttle valve in high pressure ratio CO₂ trans-critical refrigeration cycle has great potential to improve cycle efficiency. Compared with other expanders, the Wankel expander has a series of advantages in terms of compactness, economy and reduced mechanical vibrations, the working process consists of intake process, expand process and exhaust process. In this paper, the operation mechanism of Wankel expander in the refrigeration system is theoretically analyzed, including the working process, the influence of shape factor, rotor profile, intake angle, eccentric distance and wheelbase coefficient. A non-isentropic model which introduces isentropic efficiency is used to calculate the physical properties of cavity according to the change of volume. In this paper, by expanding supercritical CO₂ from 10 MPa, 313.15 K to 3.485 MPa, 273.69 K, the Wankel expander can increase the refrigeration coefficient by 23.7% in the discussed conditions; the shape factor and the intake angle are the main factors that affect the performance of Wankel expander. It is necessary to select appropriate parameters in order to optimize the efficiency.

Key words: trans-critical CO₂, refrigeration cycle, expander, Wankel engine

摘要：

在高压比跨临界CO₂制冷循环中，采用膨胀机替代节流元件对提高循环效率有较大的潜力。分析了以三角转子机械作为膨胀机的运行机理，对三角转子膨胀机的工作过程进行计算和分析，并从形状因子、转子型线、进气角度、偏心距和轴距系数入手，讨论了影响三角转子膨胀机的影响因素。在设定情况下，使用三角转子膨胀机可以通过将10 MPa、313.15 K的超临界CO₂膨胀至3.49 MPa、273.69 K，并将制冷系数提高23.7%。形状因子和进气角度是影响膨胀机运行效果的主要因素，需选取适当的参数来提高效率。

关键词: 跨临界CO₂, 制冷循环, 膨胀机, 三角转子

CLC Number:

TB 653

LI Peiyun, LI Yang, WANG Wenbin, WANG Wen. Application analysis of Wankel expander in CO₂ trans-critical refrigeration cycle[J]. CIESC Journal, 2021, 72(S1): 161-169.

李沛昀, 李杨, 王文彬, 王文. 三角转子膨胀机在跨临界CO₂制冷循环中的应用分析[J]. 化工学报, 2021, 72(S1): 161-169.

Figures/Tables 9

Fig.1 Diagram of thermodynamic process

Fig.2 Schematic diagram of a Wankel expander

Fig.3 Working principle of a Wankel expander

Fig.4 Flowchart of computation

Table 1 Computation parameters

符号	参数	数值
e	偏心距/mm	7
K	形状因子	6
B	腔体高度/mm	25
Δτ	时间步长/μs	5.556
ω	转速/( r/min)	3000
A	进排气截面积/ mm²	80
P_in	进气入口压力/ MPa	10
T_in	进气入口温度/ K	313.15
α_in	进气角度/(°)	103
η_s	等熵效率	0.4

Fig.5 Parameters variation of a Wankel expander with eccentric angle

Table 2 Influence of shape factor

K	p_out / MPa	T_out / K	x_out	q_m / (kg/s)	W_exp / W	${\overset{ˉ}{q}}_{m}$ /(kg/(mm²·s))	${\overset{ˉ}{W}}_{e x p}$ / (W/mm²)
5	3.536	273.7	0.4362	0.2121	2483	5.30×10^-5	0.6204
5.5	3.535	273.69	0.4362	0.2334	2735	4.85×10^-5	0.5686
6	3.535	273.69	0.4361	0.2546	2988	4.47×10^-5	0.5246
6.5	3.535	273.69	0.4361	0.2758	3238	4.14×10^-5	0.4864
7	3.532	273.66	0.4357	0.297	3529	3.86×10^-5	0.4585
7.5	3.529	273.62	0.4352	0.3182	3828	3.61×10^-5	0.4344
8	3.526	273.59	0.4347	0.3394	4130	3.39×10^-5	0.4128

Table 2 Influence of shape factor

K	p_out / MPa	T_out / K	x_out	q_m / (kg/s)	W_exp / W	${\overset{ˉ}{q}}_{m}$ /(kg/(mm²·s))	${\overset{ˉ}{W}}_{e x p}$ / (W/mm²)
5	3.536	273.7	0.4362	0.2121	2483	5.30×10^-5	0.6204
5.5	3.535	273.69	0.4362	0.2334	2735	4.85×10^-5	0.5686
6	3.535	273.69	0.4361	0.2546	2988	4.47×10^-5	0.5246
6.5	3.535	273.69	0.4361	0.2758	3238	4.14×10^-5	0.4864
7	3.532	273.66	0.4357	0.297	3529	3.86×10^-5	0.4585
7.5	3.529	273.62	0.4352	0.3182	3828	3.61×10^-5	0.4344
8	3.526	273.59	0.4347	0.3394	4130	3.39×10^-5	0.4128

Fig.6 Influence of profile design

Table 3 Influence of intake angle

α_in / (°)	p_out / MPa	T_out / K	x_out	q_m / (kg/s)	W_exp /W	${\overset{ˉ}{q}}_{m}$ /(kg/(mm²·s))	${\overset{ˉ}{W}}_{e x p}$ / (W/mm²)
95	3.137	269.22	0.4476	0.2206	3087	3.87×10^-5	0.5419
100	3.386	272.06	0.4404	0.2416	3037	4.24×10^-5	0.5331
103	3.535	273.69	0.4361	0.2546	2988	4.47×10^-5	0.5246
106	3.685	275.27	0.432	0.2677	2920	4.70×10^-5	0.5126
109	3.83	276.76	0.4273	0.2809	2879	4.93×10^-5	0.5054
112	3.958	278.04	0.4201	0.2943	3047	5.17×10^-5	0.5351
115	4.091	279.34	0.414	0.3079	3122	5.41×10^-5	0.5481

Table 3 Influence of intake angle

α_in / (°)	p_out / MPa	T_out / K	x_out	q_m / (kg/s)	W_exp /W	${\overset{ˉ}{q}}_{m}$ /(kg/(mm²·s))	${\overset{ˉ}{W}}_{e x p}$ / (W/mm²)
95	3.137	269.22	0.4476	0.2206	3087	3.87×10^-5	0.5419
100	3.386	272.06	0.4404	0.2416	3037	4.24×10^-5	0.5331
103	3.535	273.69	0.4361	0.2546	2988	4.47×10^-5	0.5246
106	3.685	275.27	0.432	0.2677	2920	4.70×10^-5	0.5126
109	3.83	276.76	0.4273	0.2809	2879	4.93×10^-5	0.5054
112	3.958	278.04	0.4201	0.2943	3047	5.17×10^-5	0.5351
115	4.091	279.34	0.414	0.3079	3122	5.41×10^-5	0.5481

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Application analysis of Wankel expander in CO₂ trans-critical refrigeration cycle

三角转子膨胀机在跨临界CO₂制冷循环中的应用分析

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References 22

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