Augmented Thermal Performance of Straight Heat Pipe Employing Annular Screen Mesh Wick and Surfactant Free Stable Aqueous Nanofluids

Stable surfactant-free Al₂O₃/deionized (DI) water nanofluids are prepared by a two-step process and are stabilized using an ultrasonic homogenizer. The thermal conductivity enhancement measured by a transient hot wire technique demonstrated a nonlinear relationship with increase in volume fraction of dispersed nanoparticles and attains a maximum enhancement of 15% for 1 vol% of Al₂O₃ loading in deionized water at 70°C. The stabilized Al₂O₃/DI water nanofluids were employed as the working fluid in a screen mesh wick heat pipe placed horizontally. The straight heat pipe configuration is altered for more practicality in use, with crimped edges, extended conduction lengths, and minute surface depressions. The heat pipe is tested at various levels of heat inputs and concentrations of Al₂O₃ nanoparticles. The evaporator section is heated by circulating water through a heating chamber, and the condenser section is cooled under free convection. The experimental results show an optimum reduction of 22% in the thermal resistance value using 1 vol% of Al₂O₃/DI nanofluids as compared to DI water at low heat input of 12 W. The stabilized operation of the heat pipe is observed at high heat input of 73 W and at low concentration of 0.005 vol% Al₂O₃/DI water nanofluids. The findings emphasize potential for nanofluids as future heat pipe fluids.     

An experimental study on stability and some thermophysical properties of multiwalled carbon nanotubes with water–ethylene glycol mixtures

In this study, the stability and thermophysical properties of multiwalled carbon nanotubes (MWCNTs) with double-distilled water (W) and double-distilled water/ethylene glycol (W/EG) mixtures are investigated. Stability analyses are performed through visual observation, thermal conductivity measurements, spectrophotometry and zeta potential measurement methods. An increase in ethylene glycol ratio in water increases the stability of nanofluid, which helps the nanoparticles disperse uniformly in the base fluid for a longer duration. It is concluded from the results that MWCNT nanofluids with a W/EG system (50:50) has good stability, showing no agglomeration for 36 d as compared with other nanofluids. Thermophysical properties such as thermal conductivity, viscosity and density with temperature were also measured. Maximum thermal conductivity enhancement of 29% was observed for MWCNT-nanofluid with W/EG system (50:50) at 50°C. It is also observed that with the addition of MWCNT in W/EG mixtures, viscosity and density increase but the enhancement was comparatively low with reference to thermal conductivity. From these results, it was interpreted that both stability and thermal conductivity increase with increase in ethylene glycol ratio in water.     

Heat transfer and pressure drop performance of alumina–water nanofluid in a flat vertical tube of a radiator

This work presents experimental investigation on the effects of nanofluid inlet temperature (40–90°C), Reynolds number (12,000–30,000), particle concentration (0–1 vol.%), and air velocity (0.25–0.55?m/s) on thermal and flow characteristics of water-based alumina nanofluids in a flat vertical tube of a radiator. The specific heat capacity, viscosity, density, and thermal conductivity were measured experimentally. The heat transfer coefficient enhanced (up to 31%) with an increase in fluid inlet temperature, particle volume concentration, Reynolds number as well as air inlet velocity. The pressure drop increased with an increase in the particle volume concentration and Reynolds number, while it decreased slightly with an increase in the fluid inlet temperature. The friction factor and pumping power increased with particle concentration. The friction factor decreased, while the pumping power increased with sn increase in fluid flow rate.     

Coding,evaluation, comparison,ranking and optimal selection of nanoparticles with heat transfer fluids for thermal systems

Performance of nanofluids is affected by various parameters and it is important to identify these parameters and their inheritance that determine the overall performance of nanofluids. The 125 attributes are identified and a coding scheme is developed for an in-depth understanding and visual comparison of nanofluids more precisely. A three-stage methodology named multi-attribute decision making (MADM) is used for the evaluation, comparison and ranking of nanofluids and it selects the optimum nanofluid for a given application in less time and effort. In the first stage of MADM, known as elimination search, a long list of alternatives is converged to a manageable list. Later, the procedure is followed for the ranking of nanofluids by employing the Technique for Order Preference by Similarity to Ideal Solution (TOPSIS) approach. This ensures that the selected nanofluid is the closest to the hypothetical best nanofluid and the farthest from the hypothetical worst nanofluids. Finally, the optimum nanofluid is selected by considering the cost factor as well as the government rules for a particular application. This methodology has been explained by an example in which nanofluids are developed in the laboratory to evaluate the heat transfer characteristics for cooling devices such as microchannels. This scheme is user friendly and a simple mathematical procedure is required to use it.     

Heat Transfer and Pressure Drop Characteristics of Dilute Alumina–Water Nanofluids in a Pipe at Different Power Inputs

This work addresses the effect of temperature on the thermophysical properties (i.e., density, viscosity, thermal conductivity, and specific heat capacity) of alumina–water nanofluid over a wide temperature range (25°C–75°C). Low concentrations (0–0.5% v/v) of alumina nanoparticles (40 nm size) in distilled water were used in this study. The pressure drop and the effective heat transfer coefficient of nanofluids were also estimated for different power inputs and at different flow rates corresponding to Reynolds numbers in the range of 1500–6000. The trends in variation of thermophysical properties of nanofluids with temperature were similar to that of water, owing to their low concentrations. However, the density, viscosity, and thermal conductivity of nanofluids increased, while the specific heat capacity decreased with increasing the nanoparticle concentration. The convective heat transfer coefficient of the nanofluid and the pressure drop along the test section increased with increasing the particle concentration and flow rate of nanofluid. Results showed that the heat transfer coefficient increases, while the pressure drop decreases slightly with increasing the power input. This is because of the fact that increasing power input to heater increases the bulk mean temperature of nanofluids, resulting in a decreased viscosity. The prepared nanofluids were found to be more effective under turbulent flow than in transition flow.     

Heat Transfer Studies of a Heat Pipe

The present investigation reports a theoretical and experimental study of a wire screen heat pipe, the evaporator section of which is subjected to forced convective heating and the condenser section to natural convective cooling in air. The theoretical study deals with the development of an analytical model based on thermal resistance network approach. The model computes thermal resistances at the external surface of the evaporator and condenser as well as inside the heat pipe. A test rig has been developed to evaluate the thermal performance of the heat pipe. The effects of operating parameters (i.e., tilt angle of the heat pipe and heating fluid inlet temperature at the evaporator) have been experimentally studied. Experimental results have been used to compare the analytical model. The heat transfer coefficients predicted by the model at the external surface of the evaporator and condenser are reasonably in agreement with experimental results.