Influence of nanoparticle concentrations on flow boiling heat transfer coefficients of Al_2O_3/R141b in micro heat exchanger by direct metal laser sintering

Al_2O_3/R141 b + Span-80 nanorefrigerant for 0.05 wt.% to 0.4 wt.% is prepared by ultrasonic vibration to investigate the influence of nanoparticle concentrations on flow boiling heat transfer of Al_2O_3/R141 b + Span-80 in micro heat exchanger by direct metal laser sintering. Experimental results show that nanoparticle concentrations have significantly impact on heat transfer coefficients by homogeneity test of variances according to mathematical statistics. The heat transfer performance of Al_2O_3/R141 b+ Span-80 nanorefrigerant is enhanced after adding nanoparticles in the pure refrigerant R141 b. The heat transfer coefficients of 0.05 wt.%, 0.1 wt.%, 0.2 wt.%, 0.3 wt.%and 0.4 wt.% Al_2O_3/R141 b + Span-80 nanorefrigerant respectively increase by 55.0%, 72.0%, 53.0%, 42.3% and 39.9% compared with the pure refrigerant R141 b. The particle fluxes from viscosity gradient, non-uniform shear rate and Brownian motion cause particles to migrate in fluid especially in the process of flow boiling. This migration motion enhances heat transfer between nanoparticles and fluid. Therefore, the heat transfer performance of nanofluid is enhanced. It is important to note that the heat transfer coefficients nonlinearly increase with nanoparticle concentrations increasing. The heat transfer coefficients reach its maximum value at the mass concentration of 0.1% and then it decreases slightly. There exists an optimal mass concentration corresponding to the best heat transfer enhancement. The reason for the above phenomenon is attributed to nanoparticles deposition on the minichannel wall by Scanning Electron Microscopy observation. The channel surface wettability increases during the flow boiling experiment in the mass concentration range from 0.2 wt.% to 0.4 wt.%. The channel surface with wettability increasing needs more energy to produce a bubble. Therefore, the heat transfer coefficients decrease with nanoparticle concentrations in the range from 0.2 wt.% to 0.4 wt.%. In addition, a new correlation has been proposed by fitting the experimental data considering the influence of mass concentrations on the heat transfer performance. The new correlation can effectively predict the heat transfer coefficient.

Heat Transfer in Nucleate Pool Boiling of Binary and Ternary Refrigerant Mixtures

Heat transfer coefficients in nucleate pool boiling were measured on a horizontal copper surface for refrigerants, HFC-134a, HFC-32, and HFC-125, their binary and ternary mixtures under saturated conditions at 0.9MPa. Compared to pure components, both binary and ternary mixtures showed lower heat transfer coefficients.This deterioration was more pronounced as heat flux was increased. Experimental data were compared with some empirical and semi-empirical correlations available in literature. For binary mixture, the accuracy of the correlations varied considerably with mixtures and the heat flux. Experimental data for HFC-32/134a/125 were also compared with available correlated equation obtained by Thome. For ternary mixture, the boiling range of binary mixture composed by the pure fluids with the lowest and the medium boiling points, and their concentration difference had important effects on boiling heat transfer coefficients.

Experimental Studies on the Thermal Performance of a Parabolic Dish Solar Receiver with the Heat Transfer Fluids SiC+ Water Nano Fluid and Water

An experimental investigation has been carried out with aa point focusing dish reflector of 12 square meters aperture area, exposed to the average direct normal irradiations of 810 W/m^2. This work focuses on enhancinge the energy and exergy efficiencies of the cavity receiver by minimizing the temperature difference between the wall and heat transfer fluids. Two heat transfer fluids Water and SiC + water nano fluid have been prepared from 50 nm particle size and 1% of volume fraction, and experimented separately for the flow rates of 0.2 lpm to 0.6 lpm with an interval of 0.1 lpm. The enhanced thermal conductivity of nano fluid is 0.800115 W/mK with the k_(eff)/k_b ratio of 1.1759 determined by using the Koo and Kleinstreuer correlation. The maximum attained energy and exergy efficiencies are 29.14% and 24.82% for water, and 32.91% and 39.83% for SiC+water nano fluid. The nano fluid exhibits enhanced energy and exergy efficiency of 12.94% and 60.48% than that of water at the flow rate of 0.5 lpm. The result shows that the system with SiC+Water produces higher exergy efficiency as compared to energy efficiency; in the case of water alone, the energy efficiency is higher than exergy efficiency.

Explicit analytical wave solutions of unsteady 1D ideal gas flow with friction and heat transfer

Several families of algebraically explicit analytical wavesolutions are derived for the unsteady 1D ideal gas flow with friction and heat-transfer, which include one family of travelling wave solutions, three families of standing wave solutions and one standing wave solution. \{Among\} them, the former four solution families contain arbitrary functions, so actually there are infinite analytical wave solutions having been derived. Besides their very important theoretical meaning, such analytical wave solutions can guide the development of some new equipment, and can be the benchmark solutions to promote the development of computational fluid dynamics. For example, we can use them to check the accuracy, convergence and effectiveness of various numerical computational methods and to improve the numerical computation skills such as differential schemes, grid generation ways and so on.

Computational Fluid Dynamics(CFD) Modeling of Heat Transfer in a Polymeric Membrane using Finite Volume Method

The efficiency,robustness and reliability of recent numerical methods for finding solutions to flow problems have given rise to the implementation of computational fluid dynamics(CFD) as a broadly used analysis method for engineering problems like membrane separation system.The CFD modeling in this study observes steady and unsteady(transient) heat flux and temperature profiles in a polymeric(cellulose acetate) membrane.This study is novel due to the implementation of user defined scalar(UDS) diffusion equation by using user-defined functions(UDFs) infinite volume method(FVM).Some details of the FVM used by the solver are carefully discussed when implementing terms in the governing equation and boundary conditions(BC).The contours of temperature due to high-temperature gradient are reported for steady and unsteady problems.

Study of Cycle Output Improvement by Work-Fluid Including Phase Change Material

This paper deals with the output improvement of heating and cooling cycle by using the work-fluid including phase change material.The experimental study is carried out by heat exchange between work-fluid and heat transfer surface.The work-fluid is flown to a high temperature or a low temperature heat transfer surface from the narrow path.In order to increase the amount of the heat transmission,a trace of Diethylether(boiling point 34.8 ℃),as a phase change material(PCM),is added to the work-fluid.The parameters of the experiment are additive amount of PCM,the rotational speed of the displacer piston and the temperature of heat transfer surface.It is clarified that the increasing of engine cycle output is brought by the PCM addition.The effect of PCM addition is evaluated by output ratio which is defined from the experimental cycle output data.The requirements for acquiring the increasing effect of output by adding PCM are clarified.

Convective heat transfer for incompressible laminar gas flow in micropassage with constant wall temperature

Theoretical investigations have been performed on the convective heat transfer for incompressible laminar flow of gases through microtube and parallel-plates micropassages with constant wall temperature. Considering the change in thermal conductivity and viscosity of gas in wall adjacent region from the kinetic theory, mathematical models are built for both of the micropassages. The dimensionless temperature distribution and the corresponding heat transfer characteristics are simulated numerically, and the results discussed briefly.