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In the present work, investigation on flow boiling heat transfer of R-134a inside a horizontal tube and also the tubes with coiled wire inserts has been done experimentally. The experimental setup which was used in this investigation was a well instrumented vapor compression refrigeration system. This set-up consists a test evaporator which all the experiments were carried out on it. Refrigerant which flows inside the tube of test evaporator is electrically heated by the coils around it. The evaporator tube is a copper tube with 7.5 mm internal diameter. The range of some operating parameters are: refrigerant mass velocities 54-136 kg/m2s, vapor qualities 0.2-1.0 and heat fluxes 2-6 kW/m2. The empirical data were collected for plain tube and tubes with seven different coiled wire inserts (different coil pitches and different wire diameters). The results show that the insertion of a helically coiled wire inside the evaporator tube increases the heat transfer coefficient by as much as 83% above the plain tube values on a nominal area basis. An empirical correlation was developed to predict the heat transfer coefficient during flow boiling of R-134a inside horizontal coiled wire inserted tubes.

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Abstract In the present work, investigation on flow boiling heat transfer of R-134a inside a horizontal tube and also the tubes with coiled wire inserts has been done experimentally. The experimental setup which was used in this investigation was a well instrumented vapor compression refrigeration system. This set-up consists a test evaporator which all the experiments were carried out on it. Refrigerant which flows inside the tube of test evaporator is electrically heated by the coils around it. The evaporator tube is a copper tube with 7.5 mm internal diameter. The range of some operating parameters are: refrigerant mass velocities 54-136 kg/m2s, vapor qualities 0.2-1.0 and heat fluxes 2-6 kW/m2. The empirical data were collected for plain tube and tubes with seven different coiled wire inserts (different coil pitches and different wire diameters). The results show that the insertion of a helically coiled wire inside the evaporator tube increases the heat transfer coefficient by as much as 83% above the plain tube values on a nominal area basis. An empirical correlation was developed to predict the heat transfer coefficient during flow boiling of R-134a inside horizontal coiled wire inserted tubes.

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Electrohydrodynamic (EHD) is one of the techniques for heat transfer enhancement. In current study, the enhancement of natural convection heat transfer inside a vertical tube is experimentally investigated under applying a strong electrical field (EHD). For this purpose, a wire electrode with positive polarity is used along the pipe axis while the inner surface of tube is connected to the ground. EHD disturbs the thermal boundary layer by generating ionic wind which flows from wire electrode to inner side of tube and causes the heat transfer enhancement. In this study, the effects of wire electrode diameter and also electrical field on heat transfer enhancement are investigated. Obtained data are reported as local Nusselt number along the pipe axis and mean Nusselt number. The results show that decreasing the wire electrode diameter increases the heat transfer of tube. In addition, increasing of electrical current due to strong electrical field, increases the Nusselt number. At the lowest wire electrode diameter, the highest Nusselt number was observed which was 2.03 times more than the case that no electrical field was applied.

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Experimental evaluation of overall convection heat transfer coefficient of a rectangular bar in the vicinity of a flat plate is investigated. A quad of rectangular shape and later a quad with the cause of optimal heat transfer are placed at the near and the inside of a turbulent boundary layer over a flat plate. The overall convection heat transfer coefficient of the flat plate are measured and compared to the case similar to a single flat plate. A low speed wind tunnel is employed to maintain main flow field at the requested speed and special electrical circuit is prepared to provide heat and measure heat losses from the flat plate. Conclusion is made that when the obstacle closes to the flat plate, the total convective heat transfer coefficient increases to a maximum and then reduces again by moving towards the plate. Distance from the flat in which the maximum heat transfer coefficient occurs is reported.

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In this study, forced convective heat transfer and pressure drop behavior of multi walled carbon nanotubes (CNT)-water nanofluid were evaluated under constant heat flux in a circular tube. For this purpose, first, homogeneous aqueous suspension of CNT using gum Arabic (GA) surfactant was prepared in concentrations 0.05%, 0.1% and 0.2% wt. Then, the above mentioned nanofluids were evaluated in Reynolds number range of 800-2000 under constant heat flux. The results indicate a significant increase in convective heat transfer coefficient of nanofluids with the addition of small amounts of CNT in deionized water. Also, heat transfer coefficient is enhanced with increasing concentration and Reynolds number. However, the effect of increasing concentrations of CNT is higher than the increase in Reynolds number. In addition, the pressure drop data on the different concentrations and Reynolds numbers are also investigated. At low weight concentrations of CNT, the deal of pressure drop of nanofluids containing CNT and base fluids is approximately similar and the gap between them is negligible. This means that no extra pump power is required for low concentration CNT/water nanofluid. The maximum increase in heat transfer coefficient is 42.8%, which occurred at Re=2027, and a concentration of 0.2% wt.