An introduction to heat transfer

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A User’s Guide

Solar Engineering Of Thermal Processes

A review of the applications of nanofluids in solar energy

The limitation of hot spot cooling in microchips represents an important hurdle for the electronics industry to overcome with coolers yet to exceed the efficiencies required. Nanotechnology-enabled heat sinks that can be magnetophoretically formed onto the hot spots within a microfluidic environment are presented. CrO2 nanoparticles, which are dynamically chained and docked onto the hot spots, establish tuneable high-aspect-ratio nanofins for the heat exchange between these hot spots and the liquid coolant. These nanofins can also be grown and released on demand, absorbing and releasing the heat from the hot spots into the microfluidic system. It is shown that both high aspect ratio and flexibility of the fins have a dramatic effect on increasing the heat sinking efficiency. The system has the potential to offer a practical cooling solution for future electronics.

Dynamic nanofin heat sinks

One of the most common methods used to transfer heat is by using various types of heat exchangers. The efficiency of heat exchangers can be improved through active or passive techniques. Active technique involves the use of external forces such as vibration, rotation, electric field, etc. Where else the passive techniques involve fluid additives or modified surface geometries. Helical coiled tubes have been known to exhibit excellent heat transfer characteristics compared to regular straight tubes and for this reason it is widely used in industrial applications. Another known passive technique to enhance heat transfer is by altering the fluid thermal conductivity. Recent studies have shown that nanofluids (nano sized particles suspended in conventional heat transfer fluids) posses superior heat transfer capabilities due to its high thermal conductivity. The amount of research done in this area is fairly new and limited. Most studies done on helical coiled tubes and nanofluids show enhanced heat transfer capabilities and results that challenge conventional theories and limitations on heat transfer devices and fluids. Several important aspects of HCTHE that affect the performance such as geometry, fluid inlet and outlet arrangement, and types of HCTHE are discussed based on the reported findings from experimental and numerical studies. This review also focuses on the vital aspects of nanofluids such as types, properties and heat transfer characteristics and limitations towards the application of nanofluids.

Heat transfer and fluid flow characteristics in helically coiled tube heat exchanger (HCTHE) using nanofluids: a review

In the present work, laminar cross flow forced convective heat transfer of nanofluid over tube banks with various geometry under constant wall temperature condition is investigated numerically. We used nanofluid instead of pure fluid ,as external cross flow, because of its potential to increase heat transfer of system. The effect of the nanofluid on the compact heat exchanger performance was studied and compared to that of a conventional fluid.The two-dimensional steady state Navier-Stokes equations and the energy equation governing laminar incompressible flow are solved using a Finite volume method for the case of flow across an in-line bundle of tube banks as commercial compact heat exchanger. The nanofluid used was alumina-water 4% and the performance was compared with water. In this paper, the effect of parameters such as various tube shapes ( flat, circle, elliptic), and heat transfer comparison between nanofluid and pure fluid is studied. Temperature profile, heat transfer coefficient and pressure profile were obtained from the simulations and the performance was discussed in terms of heat transfer rate and performance index. Results indicated enhanced performance in the use of a nanofluid, and slight penalty in pressure drop. The increase in Reynolds number caused an increase in the heat transfer rate and a decrease in the overall bulk temperature of the cold fluid. The results show that, for a given heat duty, a mas flow rate required of the nanofluid is lower than that of water causing lower pressure drop. Consequently, smaller equipment and less pumping power are required.

Numerical Study of Fluid Flow and Heat Transfer Enhancement of Nanofluids over Tube Bank

The problem of the laminar upward mixed convection heat transfer for thermally developing air flow in the entrance region of a vertical circular cylinder under buoyancy effect and wall heat flux boundary condition has been numerically investigated. An implicit finite difference method and the Gauss elimination technique have been used to solve the governing partial differential equations of motion (Navier Stocks equations) for two-dimensional model. This investigation covers Reynolds number range from 400 to 1600, heat flux is varied from 70 W/m2 to 400 W/m2. The results present the dimensionless temperature profile, dimensionless velocity profile, dimensionless surface temperature along the cylinder, and the local Nusselt number variation with the dimensionless axial distance Z+. The dimensionless velocity and temperature profile results have revealed that the secondary flow created by natural convection have a significant effect on the heat transfer process. The results have also shown an increase in the Nusselt number values as the heat flux increases. The results have been compared with the available experimental study and with the available analytical solution for pure forced convection in terms of the local Nusselt number. The comparison has shown satisfactory agreement. .

http://www.doiserbia.nb.rs/ft.aspx?id=0354-98360802089M

Numerical study of combined convection heat transfer for thermally developing upward flow in a vertical cylinder

In this study, forced convective heat transfer is considered in channel over a backward facing step having a baffle on the top wall. Four different geometries with different expansion ratios and different type of baffles are numerically investigated. The study clearly shows that the geometry with expansion ratio 2 and solid baffle has the highest Nusselt number compared to other geometries. Considering both Nusselt number and skin friction coefficient for all four geometries clearly illustrated an increase in average Nusselt number by increasing the expansion ratio. This study clearly shows that mounting a slotted baffle at the top wall instead of a solid baffle caused a decline in average Nusselt number. It is also found that for geometry with expansion ratio of 3 and a slotted baffle on the top of the channel, skin friction coefficient in both bottom wall and step wall has its minimal compared to other geometries.

Effect of Vertical Baffle Installation on Forced Convective Heat Transfer in Channel Having a Backward Facing Step

Purpose – The purpose of this paper is to investigate numerically the thermal and hydrodynamics performance of circular microchannel heat exchanger (CMCHE) using various nanofluids.Design/methodology/approach – The three‐dimensional steady, laminar developing flow and conjugate heat transfer governing equations of a balanced MCHE are solved using finite volume method.Findings – The results are shown in terms of temperature profile, heat transfer coefficient, pressure drop, wall shear stress, pumping power, effectiveness and performance index. The addition of nanoparticles increased the heat transfer rate of the base fluids. The temperature profiles of the fluids have revealed that higher average bulk temperatures were obtained by the nanofluids compared to water. The addition of nanoparticles also increased the pressure drop along the channels slightly. The increase in nanoparticle concentrations yielded better heat transfer rate while the increase in Reynolds number decreased the heat transfer rate.Resea...

Hussein A. Mohammed

Papers

Heat transfer and fluid flow characteristics in helically coiled tube heat exchanger (HCTHE) using nanofluids: a review

Numerical Study of Fluid Flow and Heat Transfer Enhancement of Nanofluids over Tube Bank

Numerical study of combined convection heat transfer for thermally developing upward flow in a vertical cylinder

Effect of Vertical Baffle Installation on Forced Convective Heat Transfer in Channel Having a Backward Facing Step

Thermal and hydrodynamic performance analysis of circular microchannel heat exchanger utilizing nanofluids