An Experimental Study On Condensation Of R134a In A Multi-Port Extruded Tube

In the present study, the local characteristics of pressure drop and heat transfer are investigated experimentally for the condensation of a pure refrigerant R134a in multi-port extruded tubes of 8 channels in 1.36mm hydraulic diameter and 19 channels in 0.80mm hydraulic diameter. The experimental data of frictional pressure drop (F.P.D.) and condensation heat transfer coefficient (H.T.C.) are compared with previous correlations, most of which are proposed for the condensation of pure refrigerant in a relatively large diameter tube. Considering the effects of surface tension and kinematic viscosity, new correlation of F.P.D. is developed based on the Mishima-Hibiki correlation. New correlation of H.T.C. is also developed modifying the effect of diameter in the correlation of Haraguchi et al. NOMENCLATURE A : heat transfer surface of tube Bo : Bond number Cp : isobaric specific heat d : hydraulic diameter G : mass velocity g : gravitational acceleration L Ga : Galileo number H : width of tube ( ) ξ H : function of void fraction Nu : Nusselt number P : pressure L Ph : phase change number Pr : Prandtl number q : heat flux Re : Reynolds number S : wetted perimeter length T : temperature α : heat transfer coefficient VL h ∆ : latent heat of condensation P ∆ : pressure drop f P ∆ : frictional pressure drop m P ∆ : pressure drop due to momentum change λ : thermal conductivity μ : viscosity ν : kinematic viscosity ξ : void fraction ρ : density σ : surface tension V Φ : two-phase multiplier factor tt Χ : Lockhart-Martinelli parameter Subscripts B : free convection condensation term cal : prediction exp : experiment F : forced convection condensation term L : liquid R : refrigerant S : heat sink (cooling water) V : vapor wi : inside surface wo : outside surface ( 1) Refrigerant Pump ( 2) Flow Control Valve ( 3) Mass Flow Meter ( 4) Evaporator ( 5) Test Section ( 6) Subcooler ( 7) Liquid Receiver ( 8) Filter ( 9) Constant Temperature Water Bath (10) Coolant Pump (11) Volume Flow Meter (12) Cooling Unit INTRODUCTION From the viewpoint of global environment protection, it is urgently necessary to introduce environmentally acceptable new refrigerants and improve further the performance in the refrigeration and air-conditioning systems. As one of the methods for improving the system performance, the reduction of the diameter of heat transfer tubes is taken an interest in. There are a few previous studies on the condensation heat transfer of refrigerants in small diameter tubes. Katsuta (1994) carried out experiments of R134a in several multi-port extruded tubes, and compared the local heat transfer characteristics with several correlations proposed for relatively large diameter tubes. Yang and Webb (1996) carried out experiments on the heat transfer of R12 in a horizontal multi-port extruded tube of 2.64 mm in hydraulic diameter and a horizontal multi-port extruded microfin tube of 1.56 mm in hydraulic diameter. Moser et al. (1998) proposed a correlation using the equivalent Reynolds number model, based on experimental data of heat transfer in many horizontal tubes of 4.57-12.7 mm I.D. However, it is very difficult to measure accurately the local heat transfer characteristics in a small diameter tube using the traditional methods such as the water calorimetric method, the Wilson-plot method. Accordingly, more research efforts are required to clarify the condensation process in a small diameter tube. In the present study, local characteristics of pressure drop and heat transfer are investigated experimentally for the condensation of pure refrigerant R134a in two kinds of multi-port extruded tubes made of aluminum. The present experimental results are compared with previous correlations proposed for relatively large diameter tubes. Then, based on the present experimental data, new correlations are proposed for the frictional pressure drop and heat transfer coefficient of pure refrigerant condensing in a small diameter tube. EXPERIMENTAL APPRUTUS Figure 1 shows the schematic view of the experimental apparatus. The refrigerant liquid discharged from a gear pump (1) flows into an evaporator (4) through a mass flow meter (3). The refrigerant vapor generated at the evaporator flows into a test section (5). The refrigerant condensed in the test section returns to the pump through a subcooler (6) and a liquid receiver (7). Figure 2 shows the test section, which is composed of an inlet mixing chamber, a multi-port extruded tube, and an outlet mixing chamber. Two types of multi-port extruded tubes, dimensions of which are listed in Table 1, are tested. Each test tube is 865 mm in total length and 600 mm in effective cooling length. Eight cooling water jackets are attached on both upside and downside surfaces of the test tube; the length of each jacket is 150 mm. Sixteen heat flux sensors are inserted in between the water jackets and the test tube; the length of each sensor is 75 mm. The heat flux measured with each sensor is considered as local value in the present study. The refrigerant temperature is measured with two φ 0.5 mm K-type sheathed thermocouples inserted in the inlet and outlet mixing chambers. The outer wall temperature of the test tube is measured with 16 T-type thermocouples of φ 75 m μ O.D.