Josua P. Meyer

A Review of Thermal Conductivity Models for Nanofluids

Nanofluids, as new heat transfer fluids, are at the center of attention of researchers, while their measured thermal conductivities are more than for conventional heat transfer fluids. Unfortunately, conventional theoretical and empirical models cannot explain the enhancement of the thermal conductivity of nanofluids. Therefore, it is important to understand the fundamental mechanisms as well as the important parameters that influence the heat transfer in nanofluids. Nanofluids’ thermal conductivity enhancement consists of four major mechanisms: Brownian motion of the nanoparticle, nanolayer, clustering, and the nature of heat transport in the nanoparticles. Important factors that affect the thermal conductivity modeling of nanofluids are particle volume fraction, temperature, particles size, pH, and the size and property of nanolayer. In this paper, each mechanism is explained and proposed models are critically reviewed. It is concluded that there is a lack of a reliable hybrid model that includes all mechanisms and influenced parameters for thermal conductivity of nanofluids. Furthermore, more work needs to be conducted on the nature of heat transfer in nanofluids. A reliable database and experimental data are also needed on the properties of nanoparticles.

Convective Heat Transfer Coefficients in Concentric Annuli

Abstract The geometric shape of a passages cross-section has an effect on its convective heat transfer capabilities. For concentric annuli, the diameter ratio of the annular space plays an important role. The purpose of this study was to determine to what extent research has been done on convective heat transfer in smooth concentric annuli and, if possible, to improve on or contribute to existing theories. It was found that although various correlations exist, they are not in good agreement. For this study, experiments were conducted with a wide range of annular diameter ratios. The Wilson plot method was used to develop a convective heat transfer correlation for annular diameter ratios of 1.7 to 3.2. For Reynolds numbers (based on the hydraulic diameter), in the range of 4000 to 30000, the deduced correlation predicted Nusselt numbers accurately within 3% of experimental values.

The viscosity of nanofluids : a review of the theoretical, empirical, and numerical models

The enhanced thermal characteristics of nanofluids have made it one of the most raplidly growing research areas in the last decade. Numerous researches have shown the merits of nanofluids in heat transfer equipment. However, one of the problems is the increase in viscosity due to the suspension of nanoparticles. This viscosity increase is not desirable in the industry, especially when it involves flow, such as in heat exchanger or microchannel applications where lowering pressure drop and pumping power are of significance. In this regard, a critical review of the theoretical, empirical, and numerical models for effective viscosity of nanofluids is presented. Furthermore, different parameters affecting the viscosity of nanofluids such as nanoparticle volume fraction, size, shape, temperature, pH, and shearing rate are reviewed. Other properties such as nanofluid stability and magnetorheological characteristics of some nanofluids are also reviewed. The important parameters influencing viscosity of nanofluids are temperature, nanoparticle volume fraction, size, shape, pH, and shearing rate. Regarding the composite of nanofluids, which can consist of different fluid bases and different nanoparticles, different accurate correlations for different nanofluids need to be developed. Finally, there is a lack of investigation into the stability of different nanofluids when the viscosity is the target point.

Combined Numerical Optimization and Constructal Theory for the Design of Microchannel Heat Sinks

This study deals with the geometric optimization of a silicon based microchannel heat sink using a combined numerical optimization and constructal theory. The objective is to minimize the wall peak temperature subject to various constraints. The numerical simulations are carried out with fixed volumes ranging from 0.7 mm3 to 0.9 mm3 and pressure drop between 10 kPa to 60 kPa. The effect of pressure drop on the optimized aspect ratio, solid volume fraction, hydraulic diameter, and the minimized peak temperature are reported. Results also show that as the dimensionless pressure drop increases the maximized global thermal conductance also increases.

Horizontal two-phase flow characterization for small diameter tubes with a capacitance sensor

Two-phase flow modelling is strongly dependent on flow patterns. For the purpose of objective flow pattern identification, a capacitance sensor was developed for horizontal two-phase flow in small diameter tubes. Finite element simulations were made during design to study the effect of vapour distribution, wall thickness and electrode angle. A test rig was constructed and a series of experiments was done with horizontal air–water flow in a 9 mm tube. The sensor test results are presented in time, amplitude and frequency domain. Flow regime characterization with the capacitance measurements is clearly possible.

Economic potential of vertical ground-source heat pumps compared to air-source air conditioners in South Africa

This study was undertaken to compare the economic viability in Johannesburg, R.S.A., of vertical ground-source and air-source systems. Capital costs and optimum borehole length of the ground system were determined. Monthly heating and cooling capacities, and COPs were evaluated to determine running costs. Payback periods, net present values and internal rates of return were computed. The vertical ground-source system was marginally cheaper than the air-source counterpart.

Single-Phase Heat Transfer and Pressure Drop of the Cooling of Water inside Smooth Tubes for Transitional Flow with Different Inlet Geometries (RP-1280)

Design constraints and energy requirements have often led to heat exchangers operating outside of their design parameters. These parameters often involve the exchanger operating in the transition region of flow. Adiabatic as well as diabatic experiments were conducted inside smooth tubes with diameters of 15.88 mm (5/8 in.) and 19.02 mm (3/4 in.). Four inlet profiles were investigated; hydrodynamically fully developed, square-edged, re-entrant, and bellmouth. The test fluid was water that was cooled, with Reynolds numbers ranging between 1000 and 20,000, Prandtl numbers between 4 and 6, and Grashof numbers in the order of 105. Adiabatic results showed that transition from laminar to turbulent flow was strongly dependent on the inlet profile, with transition being delayed to Reynolds numbers as high as 12,000, confirming results of previous studies. Diabatic heat transfer and friction factor results showed that transition was independent of the inlet, with transition occurring at a Reynolds number of approximately 2100. This was due to the secondary flow suppressing the disturbance of the inlets. Laminar heat transfer and friction factors were also substantially higher than when compared with their theoretical counterparts. This could also be attributed to secondary flows, confirming previously published results. A direct relationship between friction factor and heat transfer exists and is shown to predict 88% of the friction factor data to within 15%, with a mean absolute error of 8.7% when using well-known laminar and turbulent heat transfer correlations.

Flow Visualization and Flow Pattern Identification With Power Spectral Density Distributions of Pressure Traces During Refrigerant Condensation in Smooth and Microfin Tubes

Reference LTCM-ARTICLE-2005-010doi:10.1115/1.1857942View record in Web of Science Record created on 2005-07-06, modified on 2017-05-10

A techno-economic analytical comparison of the performance of air-source and horizontal-ground-source air-conditioners in South Africa

In this paper, a comparison is made of the economical viability in South Africa of horizontal-ground-source systems and air-source systems. This study is undertaken because South African manufacturers of air-source systems are considering producing ground-source systems, since the latter systems are gaining more acceptance for environmental reasons. Monthly heating/cooling capacities and coefficients of performance for both systems are determined. Furthermore, it is found that overseas data concerning the optimum depth for the placement of a horizontal water-loop for a ground-source system are not valid for South African conditions, due to climatic differences. Therefore, a techno-economic analysis is conducted to determine the optimum depth. The payback period, net present value and internal rate of return of ground-source systems at various depths are calculated for Pretoria, South Africa. It is conclude that ground-source systems are more viable than air-source systems. It is also observed that the optimum depth for the horizontal water-loop is very shallow compared to recommendations in literature.

Wind Speed Characteristics and Resource Assessment Using Weibull Parameters

The study utilized wind speed measurements made at three heights and the Weibull parameters to study the wind speed characteristics and assess the wind power potential of seven sites in Saudi Arabia. Weibull shape and scale parameters were estimated using maximum likelihood method. These parameters were found to fit the actual wind frequency distributions with acceptable coefficient of determination (>0.95) for all the sites considered in this study. The annual mean wind speed varied between 4.30 m/s and 5.9 m/s at 40 m above ground level corresponding to Gassim and Dhulom data collection stations. The local wind shear exponent calculated using measured wind speed values at 20, 30, and 40 m and the power law were established for future use and were found to vary from 0.06 to 0.34 corresponding to Gassim and Yanbo, respectively. The Weibull shape and scale parameters increased more at 30 m compared to at 40 m with increase in height from 20 to 30 m and 30 to 40 m. No regular monthly trends could be detected whereas monthly mean wind speed, shape, and scale parameters, most probable wind speed, and maximum energy carrying wind speed was concerned. The most windy sites (Dhulom, Arar, Juaymah, Rawdat Ben-Habbas, and Dhahran) were suggested for wind power development in Saudi Arabia.