Investigation of boiling heat transfer coefficient and pressure drop in horizontal microfin tubes

Two phase flows have been preferred flow type in heat exchanger design because of their higher heat transfer coefficient compared to single phase flow. In addition, increment surface area of flow is one of the most applied techniques among the passive heat transfer enhancement techniques. The heat transfer coefficient and pressure drop increases with the increment of surface area. In this study, the increment in heat transfer coefficient and pressure drop with the usage of microfin tube was experimentally investigated for flow boiling conditions and correlations were developed in order to determine convective heat transfer coefficient and frictional pressure drop. The thesis consists of six main chapters and the studies and results performed by previous researchers was presented in the first chapter. The fundamental expressions, dimensionless numbers and correlation development method were mentioned in the second chapter. Moreover, regression analysis methods is presented and the parameters which is used in this analysis were identified. Experimental setup design, measurement equipments, experimental procedure, calculation method for experimental data and geometrical specifications belonging to microfin tube were given in the third chapter. Validation experiments carried out with liquid R134a, flow boiling heat transfer and pressure drop experiments were given in the fourth chapter. The effect of mass flux (190-381 kg/m2s1), vapor quality (0.21-0.77), saturation temperature (15-22ᵒC) and heat flux (10-15 kW/m2) on boiling heat transfer coefficient and pressure drop is presented and the results of smooth and microfin tubes compared for the same experimental conditions. The experiments revealed that heat transfer coefficient is increased up 1.9 times and pressure drop is increased up to 2.3 times. Heat transfer coefficient ratio, pressure drop ratio, heat transfer coefficient per pressure drop and enhancement factor are considered as performance evaluation criteria. Moreover, the cost of smooth and microfin tubes usage in an evaporator was determined and evaluated by using annual cost method. Different type of heat transfer coefficient and frictional pressure drop correlations are developed for flow boiling conditions by using experimental results in this chapter. Uncertainty analysis was performed for experimental parameters and they are given in the fifth chapter. The conclusion and recommendations are given in the sixth chapter.