Modeling of a Two-Phase Region Length of the Condensation of R134a and R404A Refrigerants in Pipe Minichannels With Periodic Hydrodynamic Instabilities

Abstract

The heat removal process of a compact condenser in a refrigeration compressor system, constructed on the basis of pipe minichannels, occurs in three regions: the removal region of superheat, the two-phase proper condensation region, and the liquid subcooling region. The intensity of heat removal in the two-phase region depends on the heat efficiency of the condenser. The length of this region is determined by the process parameters and the external influences. This paper analyzes the influence of periodically generated hydrodynamic disturbances on the length of the condensation region for R134a and R404A refrigerants. Experimental tests were conducted for six diameters of pipe minichannels: 0. 64, 0.90, 1.44, 1.92, 2.30, and 3.30 mm. A range of frequencies from 0.2 to 5 Hz of periodically generated disturbances showed an explicit and unfavorable influence on the decrease of the length of the proper condensation region. This influence reduced the efficiency of the process. A computational model was developed to calculate the length of the two-phase region under disturbance conditions. The model proposed was verified by the results of the experiment, and compliance was obtained in the range of ±15%.

Modelling of a two-phase zone during condensation of R1234yf refrigerant in tubular minichannels under periodic dynamic disturbances

“…Single phase cool-down and sub-cooling sub-zones are characterized by much lower heat transfer coefficients. The onset of instabilities, both static and dynamic, may cause the disturbance of distribution of sub-zones inside the condenser, including a shrinkage of the active area of proper condensation characterized by the highest heat transfer coefficient [12]. Fig.…”

“…Diagram of the distribution of the temperature of refrigerant and cooling medium during condensation inside the horizontal pipe of condenser; T p -superheated vapour temperature, T s -temperature of condensation (saturation), T ffinal temperature of the process, T in ., T out . -temperature of cooling medium, at the beginning and end of process respectively, PPS -starting point of condensation [12].…”

“…The exact state of the system becomes probabilistic or stochastic. The flow instabilities in two-phase media (liquid-gas) propagate in a form of two phenomena: a pressure wave (acoustic wave travelling with the speed of sound), flow rate fluctuations (due to the oscillations of void fraction) and temperature instability also known as the evaporation front (for the evaporation phase change) or condensation front (for the condensation phase change) [12][13][14][15][16][19][20][21][22][23].…”

“…Previous experimental research on influence of instabilities on flow phase change of refrigerants in minichannels allowed partial recognition of those phenomena [1][2][3][4][5][6][12][13][14][15][16]. However, there are no publications that concern those phenomena regarding new pro-ecological refrigerants that are meant as a substitutes for those being phased out.…”

Two-phase flow phenomena assessment in minichannels for compact heat exchangers using image analysis methods

“…The research carried out for the minichannels is still rising. For example, the analysis of the influence of periodically generated hydrodynamic disturbances in mini compact condenser, with the diameter of channels in the range of 0.64-3.30 mm [39]. Another example is the investigation of frictional pressure drop during adiabatic liquid-vapour flow inside minichannels with hydraulic diameters ranging from 0.96 mm to 2.00 mm [40].…”