Düşey dar kanallarda R134a akışkanının yoğuşmasının deneysel incelenmesi

Özet

Herhangi bir ısıl sistemde faz değişimi olması halinde, olmaması haline göre daha fazla ısı geçişinin gerçekleşmesi, araştırmacıları bu alanda çalışma yapmaya sevk etmiştir. Bu husus göz önünde tutularak bu çalışmada; bir faz değişimi olan yoğuşma, deneysel olarak incelenmiştir. Deneysel çalışma kapsamında kurulan deney tesisatında; R134a soğutucu akışkanının, aralarında silindirik tüpün de bulunduğu 5 farklı düşey mini kanaldaki laminer yoğuşması incelenmiştir. Deneylerde kullanılan test üniteleri; silindirik tüp ile 4 adet çok-girişli kanallardır. Pratikte bundy boru olarak adlandırılan silindirik kanalın iç çapı 3.3 mm olup, üzeri bakır kaplı çelikten imal edilmiştir. Çokgirişli kanallar ise; farklı hidrolik çaplarda olmak üzere, 3-girişli, 5-girişli, 6-girişli ve 14-girişli olup alüminyumdan imal edilmiş kanallardır. Söz konusu bu çok-girişli kanalların hidrolik çapları sırasıyla, 4.74 mm, 3.6 mm, 1.53 mm ve 1.02 mm’dir. Böylece; kanal geometrisinin ve hidrolik çapının, R134a yoğuşmasında ısı geçişi ve basınç düşümü üzerindeki etkisi deneysel olarak tespit edilmiştir. Deneyler; farklı doyma sıcaklıkları ve farklı kütlesel akılar için tekrarlanmıştır. Böylece; düşey kanallardaki R134a soğutucu akışkanının yoğuşması, farklı doyma sıcaklıkları ve farklı kütlesel akılar için karşılaştırılabilmiştir. Ayrıca; bütün kanallardaki deneysel sonuçlardan hareketle, soğutucu akışkanların yoğuşması ile ilgili olarak literatürde yaygın bir şekilde yer alan korelasyonlara benzer bir korelasyon geliştirilmiştir. Yoğuşmada Nu sayısı için geliştirilen bu boyutsuz korelasyonun, literatürdeki korelasyonlar ile oldukça uyumlu olduğu görülmüştür.

Abstract

In any thermal system, the fact that more heat is transferred in the occurrence of phase change is a motivation for the researchers to work on. Furthermore, it has become inevitable to build smaller (more compact) heating and cooling systems, while the technology is improving more and more. This resulted in the necessity of the occurrence of phase changes in smaller spaces. Therefore it is required to know the mechanisms of phase change in narrow spaces and to improve heat transfer. Taking these facts into consideration, this study is performed to investigate condensation, which is a frequently encountered phase change in thermal systems, in experimental aspect. In the experimental study, laminar condensation of coolant R134a is investigated for five different vertical mini channels including the vertical cylindrical tube and the other four multi-port channels. The cylindrical channel with an inner diameter of 3.3 mm is made of steel coated with copper. The other four multi-port mini channels, which have 3 ports, 5 ports, 6 ports and 14 ports, are made of aluminum. The hydraulic diameters of these multi-port channels are 4.74 mm, 3.6 mm, 1.53 mm and 1.02 mm, respectively. In this way, the effects of channel configuration and hydraulic diameter on heat transfer and pressure drop for R134a condensation are determined experimentally. In the experimental investigation, the average values of thermal parameters and pressure drop are obtained along the test unit. The experiments are repeated for different saturation temperatures (therefore different saturation pressures) and for different mass fluxes. Condensation data are taken for refrigerant R134a at 32 °C and 42 °C saturation temperatures for all mini channels. Therefore, it has been possible to compare the condensation of coolant R134a in cylindrical and multi-port channels for different saturation temperatures and different mass fluxes. The experimental setup established for this study consists of the following twelve main items: coolant pump, flowmeter to measure the flow rate of the coolant R134a, pre-heating section, heating section (superheater), condensation unit, a transparent Plexiglass heat exchanger covering the condensation unit, a rotameter to measure the flow rate of the cooling water, pre-cooling section, cooling section, a receiver, a thermostatic reservoir for cooling water supply and a data acquisition system. As shown in Figure 1, refrigerant R134a enters the test section at a known vapor quality. It is condensed in the test section against the cooling water flowing in the annulus. The two-phase mixture leaving the test section enters a post- condenser. The subcooled liquid is then passed through a receiver which is in a cooler and the coolant pump. The pump moves the liquid to the pre-heater and the electric post-heater (superheater) which fixes the test section inlet vapor quality. The flowmeter is placed between the pump and the pre-heater to measure refrigerant flow rate in liquid phase. The refrigerant flow rate can be independently controlled by the pump. The inlet vapor quality is determined by the heat input to the electric post heater which can be independently controlled. The test section saturation temperature is controlled by adjusting the cooling water flow rate. The experimental results are investigated in two parts, thermal and hydrodynamic analysis. In the thermal analysis, graphs are obtained for the variation of average heat transfer coefficient with respect to average steam quality. As a result, it is shown that the average heat transfer coefficient increases when the average steam quality is increased. Also, graphs are obtained for the variation of average Nu number with respect to equivalent Re number. Similarly as a result, it is shown that the average Nu number increases when the equivalent Re number is increased. In the hydrodynamic analysis part, graphs are obtained for the variation of the pressure loss due to friction during condensation with respect to average steam quality. Also, other graphs are obtained for the two-phase friction coefficient during condensation with respect to equivalent Re number. As a result, mathematical relations are developed between ftp and Reeq for all mini channels. It is found that the two-phase friction coefficient decreases when the Reeq number is increased similar to Moody chart. As a result, a simple dimensionless correlation is developed for Nu number in refrigerant condensation which is in good agreement with the correlations in literature.