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A numerical simulation of convective heat transfer coefficient (hconv) was studied with Cu-Water and TiO2-Water nanofluids flowing through a circular tube subjected to uniform wall heat flux boundary conditions under laminar and turbulent regimes. Four different concentrations of nanofluids (ɸ = 0.5, 1, 1.5 and 2%) were used for the analysis and the Reynolds number (Re) was varied from laminar (500 to 2000) to turbulent flow regime (5000 to 20,000). The dependence of hconv on Re and ɸ was investigated using a single-phase Newtonian approach. In comparison to base fluid, average hconv enhancements of 10.4% and 7.3% were noted, respectively, for the maximum concentration (ɸ = 2%) and Re = 2000 for Cu-Water and TiO2—water nanofluids in the laminar regime. For the same ɸ under the turbulent regime (Re = 20,000), the enhancements were noted to be 14.6% and 13.2% for both the nanofluids, respectively. The random motion (Brownian motion) and heat diffusion (thermophoresis) by nanosized particles are the two major slip mechanisms that have more influence on the enhancement of hconv. In addition, the Nusselt number (Nu) of the present work was validated for water with the Shah and Dittus Boelter equation and found to have good agreement for both the regimes. [ABSTRACT FROM AUTHOR]

The thermal performance of a phase change material (PCM) heat pipe system is experimentally analysed using acetone as heat pipe fluid in a heat load range of 10-50 W at different flow rates of the condenser coolant. The evaporator of the heat pipe is enclosed in a chamber which filled with a PCM or water. Heat inputs are applied at the evaporator of the heat pipe through the PCM or water. In this study, the heat retention as well as cooling time of the PCM-water are estimated at different heat loads and flow rates of condenser coolant. Similarly, the thermal resistance, evaporator and condenser heat transfer coefficients are also estimated at different heat loads. It is observed that the PCM takes more time during heating and cooling cycles to reach the steady-state temperatures and the temperature values reached during heating are also higher for PCM compared to water. The use of PCM enhances the thermal storage capacity and shows a maximum enhancement of 200% in heat retention time compared to water at 50 W. Moreover, a maximum enhancement of 63.6% is observed in the steady-state temperature of the PCM compared to water. Similarly thermal resistance, evaporator wall temperature and heat transfer coefficients of the heat pipe also vary for PCM and water. The experimental results indicate that PCM or water can be used in this combined system depending upon requirement of thermal storage or electronics cooling. [ABSTRACT FROM AUTHOR]

This paper presents new selection and correction factor charts for the sizing of adiabatic helical capillary tubes operating with alternative refrigerants. The mathematical model is based on the conservation of mass, energy, and momentum of fluids in capillary tubes. In the two-phase flow region, the developed model is based on the homogenous flow assumption. It is verified by the accuracy of the results of comparison between simulation results and experimental data in open literature. After the developed model is validated, selection and correction charts that include the important parameters are proposed for sizing adiabatic helical capillary tube. The present charts are proposed for R-134a, R-404A, R-407B, R-407C, and R-410A, and a wide range of operation. These newly developed selection and correction charts can be used to select helical capillary tube size from flow conditions and the mass flow rate or to predict the mass flow rate from capillary tube size and flow condition. [ABSTRACT FROM AUTHOR]

Gasification and combustion of de-oiled Pongamia Pinnata seed cake is done to produce higher energy biomass waste heat "syngas" for generating power using hybrid thermoelectric generator (TEG). A test rig is fabricated and experiments conducted with synthetic oil (Therminol-55) as the heating fluid under water and aircooled methods. The hot side temperature is varied from 200 - 250°C while the coolant temperature is maintained at 30 oC for both water and air respectively. Experimental results showed 22.27% enhancement in electric power at a constant hot side temperature of 250 °C under water cooled method. In addition, simulation results for the above mentioned conditions using artificial neural networks (ANN) tool in MATLAB also agreed well with the sample experimental results. The performance parameters such as open circuit voltage, maximum output power and matched load resistance are forecasted using ANN upto maximum possible hot side temperature of 350°C. Further, the financial evaluation of Biomass gasified-thermoelectric system ($0.0018/kWh and in terms of Indian currency is Rs 0.0676/kWh) is found to be almost negligible on comparison with other available renewable energy technologies. [ABSTRACT FROM AUTHOR]

The effect of nanoparticle coating on the performance of a miniature loop heat pipe (mLHP) is experimentally investigated for heat inputs of 20'380 W using distilled water as the working fluid. Applications include the cooling of electronic devices such as circuit breaker in low voltage switch board and insulated gate bipolar transistor. Physical vapor deposition method is used to coat the nanoparticles on the evaporator surface for different coating thicknesses of 100 nm, 200 nm, 300 nm, 400 nm, and 500 nm, respectively. An optimum filling ratio (FR) of 30% is chosen for the analysis. Experimental findings show that the nanoparticle coating gives a remarkable improvement in heat transfer of the heat pipe. An average reduction of 6.7%, 11.9%, 17.2%, and 22.6% in thermal resistance is observed with coating thicknesses of 100 nm, 200 nm, 300 nm, and 400 nm, respectively. Similarly, enhancements in evaporator heat transfer coefficients of 47%, 63.5%, 73.5%, and 86% are noted for the same coating thicknesses, respectively. Evaporator wall temperature decreased by 15.4 °C for 380 W with a coating thickness of 400 nm. The repeatability test ensures the repeatability of experiments and the stability of coatings in the long run. [ABSTRACT FROM AUTHOR]

The effects of operating conditions on the air-side heat transfer, and pressure drop of a micro-channel heat exchanger under wet surface conditions were studied experimentally. The test section was an aluminum micro-channel heat exchanger, consisting of a multi-louvered fin and multi-port mini-channels. Experiments were conducted to study the effects of inlet relative humidity, air frontal velocity, air inlet temperature, and refrigerant temperature on air-side performance. The experimental data were analyzed using the mean enthalpy difference method. The test run was performed at relative air humidity's ranging between 45% and 80%; air inlet temperature ranges of 27, 30, and 33 0C, refrigerant-saturated temperatures ranging from 18 to 22 0C, and Reynolds numbers between 128 and 166. The results show that the inlet relative humidity, air inlet temperature, and the refrigerant temperature had significant effects on heat transfer performance and air-side pressure drop. The heat transfer coefficient and pressure drop for the micro-channel heat exchanger under wet surface conditions are proposed in terms of the Colburn factor and Fanning factor. [ABSTRACT FROM AUTHOR]

Experiments were conducted to study the thermal performance of meshed wick heat pipe by varying the working fluid and heat input. In this work four screen mesh wicked heat pipes were fabricated and tested. All the heat pipes were tested for heat input from 50 W to 250 W each with an increment of 50 W in each step. The heat input range selected in this study is commonly encountered in most of the electronic application devices. The thermal resistance of all the heat pipes charged with different working fluids such as de-ionised water, Al2O3/de-ionised water nanofluid of volume concentration 0.1%, and hybrid nanofluid volume concentration 0.1%, with two different combinations of (Al2O3 50%-CuO 50%)/de-ionised water and (Al2O3 25%-CuO 75%)/de-ionised water, was determined. The maximum percentage reduction was found to be 58.87% for the hybrid nanofluid of (Al2O3 25%-CuO 75%)/de-ionised water compared to base fluid. An important observation from the study is that, use of hybrid nanofluid can raise the operating range of the heat pipe beyond 250 W which makes hybrid nanofluid as a potential substitute for the conventional working fluid. [ABSTRACT FROM AUTHOR]

The paper presents the enhancement in the operational limits (boiling, entrainment, sonic, viscous and capillary limits) of heat pipes using silver nanoparticles dispersed in de-ionized (DI) water. The tested nanoparticles concentration ranged from 0.003 vol. % to 0.009 vol. % with particle diameter of <100 nm. The nanofluid as working fluid enhances the effective thermal conductivity of heat pipe by 40%, 58%, and 70%, respectively, for volume concentrations of 0.003%, 0.006%, and 0.009%. For an input heat load of 60 W, the adiabatic vapor temperatures of nanofluid based heat pipes are reduced by 9 °C, 18 °C, and 20 °C, when compared with DI water. This reduction in the operating temperature enhances the thermophysical properties of working fluid and gives a change in the various operational limits of heat pipes. The use of silver nanoparticles with 0.009 vol. % concentration increases the capillary limit value of heat pipe by 54% when compared with DI water. This in turn improves the performance and operating range of the heat pipe. [ABSTRACT FROM AUTHOR]