Comparison of constant and variable discharge flow and force coefficients for reciprocating compressor

Abstract

Enhancing the mathematical models always have been a critical issue in every field, and the reciprocating compressors are not exceptional. Flow and force coefficients are two empirical factors that help to calculate the mass flow rate through the valve and gas force, respectively. These coefficients can be considered constant values or valve lift dependent. However, in the discharge process, the piston is close to the top dead center, which may affect the discharge flow field and, subsequently, the coefficients.These coefficients should be determined by experiments or numerical simulations such as computational fluid dynamics (CFD). Experiments are expensive, and the exact geometry may not always be available to perform the CFD analysis. Also, CFD analysis for various valve lifts and piston positions is computationally expensive.This study studied the effect of constant, single variable (valve lift dependent) and double variable (valve lift and piston position dependent) coefficients on compressor performance and discharge valve movement in three operating conditions. A mathematical model was developed for this purpose, and a 3-D CFD simulation was used to compute the coefficients. To validate the results, a strain gauge and a calorimeter were used as measurement tools.The single and double variable models captured the valve behavior, and pressure picks better than the constant model. However, the constant model estimated the indicated power, cooling capacity, and COP with good accuracy. In predicting the discharge losses, the double variable model was the most reliable among the others.