Heat Transfer Between Gas and Cylinder Wall of Refrigerating Reciprocating Compressor

An investigation was made to determine the temperature dis trih i.ltion on the cylinder wall of a reciprocating refrigeration compressor and the coefficient of heat transfer between the cylinder wall and the gaseous refrigerant. The purpose was to study the working process of a refrigerating compressor and to find correlations that can be applied in computer modeling of this type of compressor. To determine the coefficient of heat transfer and the temperature distribution on the cylinder wall theoretically is rather difficult. Usually they are determined experimentally. The temperature distribution on the cylinder wall was measured at different pressure ratios,different suction temperatures and different speeds. The correlations are obtained by using the least square method. The heat flo\·: rate can be calculated b'ased on the differential equation for the first law of thermodynamics, the data of P-lp diagram and the data of valve displacement using the method of orthogonal polynomial fitting. Investigation has sho>lil that the temperature distribution on the cylinder wall varies with the pressure ratio,suction temperature, speed, oil temperature and \'lith location. Correlations were developed. INTRODUCTION Over the years,computer simulation of refrigeration reciprocating compressor has developed rapidly,and is becoming more and more powerful tool for analysing and improving the characteristics of refrigerating reciprocating compressors. Most compressor modeling desingers pay attention to the valve moving modeling,but pay little attention to the heat transfer modeling. Of course, a correct valve behavior is necess~ 110 ary for a good compressor modeling,but it is also important for the heat transfer. A correct heat transfer behavior not only influences the compression and re-expansion process,but also influences the suction and discharge process. From the thermodynamic point of view,heat transfer from the gas to the surrounding during the working processes will reduce the compression work. Therefore,the EER of compressor will be improved. It is our direct aim to determine correlations which discribe the temper~ture distribution on the cylinder wall and the heat flux through the cylinder wall for a certain type of compressor. But our indirect and more significant aim is to use our reciprocating refrigerating compressor modeling, which is developed by using our heat transfer correlation,to do other investigations. The work described in this paper is the first step of our project. On each fixed point of the cylinder wall, temperature is variable with time. Experimentlll shows that they are only variable within l"C. OWing to the thermal inertia of the cylinder material, its temperature variation is smaller then the gaseous temperature variation in the cylinder. So,temperature gradients on the cylinder wall, in our case,are taken as the average values during one cycle. The experimental compressor is a Chinese made Rl2 high speed refrigeration compressor. It has two cylinders, 100 MM bore and 70 MM stroke. The crank shaft speed is 1440 R.P.M •• EQUATIONS USED TO DETERMINE THE HEAT FLOW RATE Let us analyse a control volume shown in Fig. 1. According to the first law of thermodynamics a= au + :r ffih . ~rim + w at out ~n (l) using the ideal gas equations and negelecting the gas leakage,we get: during the compression and re-expansion process dQ __ l d(pV) dV d! f (k-1) d 'P + pdtp during the suction process dQ l d(pV) dV dms dtp = (k-1) --;;-;p+ P dlf dip ~Ts during the discharge process dQ l d(pV) dV dmd = ----+ p+ -:::c T dtf (k-1) d! f d! fl d<(l p d (2)