Local Heat Transfer Enhancement Research Articles

Heat transfer enhancement in a vertical channel with asymmetric isothermal walls by local blowing or suction

In this article, local heat transfer enhancement in a vertical parallel channel with asymmetric isothermal walls is numerically investigated. A porous wall segment is embedded in the hot wall to achieve this enhancement. By assuming the reference pressure and temperature on the other side of the hot wall, the heat transfer is enhanced as hot fluid is sucked from the channel. The heat transfer improvement is a local effect that becomes more effective with the increase of porosity and the distance measured from the entrance to the position of porous wall segment. It is shown that the local cooling enhancement is effective in low Reynolds number cases (Re ≤ 100) but is less impressive when the Reynolds number is higher. The numerical results also show the possibility of embedding multiple porous segments in a single channel, provided that the mass fraction sucked out of the channel is limited.

Local enhancement of heat transfer in a particulate cross flow—II Experimental data and predicted trends

Results from an experimental investigation of the effect of solid particles on heat transfer at the first row of a staggered tube array in cross flow are compared with those determined for equivalent conditions from correlations derived previously by the author. Comparison of the experimental data with the predicted levels of heat transfer enhancement from a range of heat transfer mechanisms suggests that the transport of thermal energy by rebounding particles is responsible for much of the measured increase in heat transfer. The total enhancement based on this model of the suspension flow compares well with the experimental data for particles of 46 and 58 μ m dia, although the agreement is not as good for 127 μm dia particles. The fine particle model of suspension flow gives levels of enhancement of the correct order for some flow conditions but fails to predict the effect of Reynolds number and particle size. In the light of these results, the suspension heat transfer mechanisms for tubes in cross flow are re-assessed and the dominant effects on enhancement are identified for specific flow conditions.

Local enhancement of heat transfer in a particulate cross flow—I Heat transfer mechanisms

The mechanisms associated with the enhancement of heat transfer over the front of a tube in a gas-particle cross flow are investigated. A correlation derived in a previous study of enhancement from the increased thermal capacity of the suspension is modified to take account of local particle residence times and the calculations of gas-to-particle heat transfer are refined. The influence on the enhancement of conduction between an impacting particle and the tube wall is analysed and the correlation is modified to include the effect of energy transport by rebounding particles. In addition, the effects of changes in the gas flow structure and boundary-layer characteristics on local heat transfer are explored.

Enhancement of Heat Transfer to a Non-Azeotropic Refrigerant Mixture in a Horizontal, Spirally Grooved Evaporator Tube.

An experiment was performed to investigate the enhancement of local heat transfer to a boiling or evaporating non-azeotropic refrigerant mixture of HCFC22 and CFC114 in a horizontal, spirally grooved tube. There was no notable difference in the enhancement among the mixture and the pure refrigerants at a high flow rate. At a low flow rate, the heat transfer to the mixture in the high quality region was found to be enhanced over the smooth tube as effectively as to the pure refrigerants ; therefore, no features of the mixture were recognized. In the low quality region where the effective enhancement was also achieved for the pure refrigerants, however, the enhancement for the mixture was greatly degraded. This degradation was theoretically explained by taking account of the variation of composition in the meniscus liquid film formed in the grooves on the upper part of the tube perimeter.

Enhancement and Control of Local Heat Transfer in a Gas Flow Containing Soft Magnetic Particles.

A novel technique for enhancing and controlling local heat transfer coefficients from the hot wall to the cooling gas flow using soft magnetic solid particles has been proposed in order to eliminate the disadvantages of inefficient gas-solid two-phase flow. The characteristics of the flow and local heat transfer were examined in a vertical rectangular channel flow involving ferrite particles of 145 μm in diameter. The Reynolds number of the channel flow was set at approximately 104 and the particle mass loading ratio was varied up to 1.6. The result indicated 60 percent enhancement of heat transfer downstream from the location of the magnet at a mass loading ratio of about 1.6. Further enhancement in heat transfer is provided by increasing the particle mass loading ratio and the magnetic field intensity. The mechanism of heat transfer enhancement can be clarified by considering the increase in the local heat capacity near the heated plate.

Influence of variable viscosity of mineral oil on laminar heat transfer in a 2:1 rectangular duct

An experimental study of laminar flow heat transfer in a 2:1 rectangular duct to mineral oil is carried out. The Hl thermal boundary condition corresponding to axially constant heat flux and peripherally constant temperature is adopted for three different heating configurations : (1) top wall heated, other walls adiabatic ; (2) bottom wall heated, other walls adiabatic ; (3) top and bottom walls heated, side walls adiabatic. Corresponding experiments are carried out using water as the test fluid. The experimental results for water show that when the upper wall is heated the influence of buoyancy force is minimal. However, in the case of oil the temperature-sensitive viscosity results in enhanced heat transfer as compared to water due to the distortion of the oil velocity profile. When the lower wall is heated, the influence of both buoyancy force and the stress differences of the mineral oil caused by the variable viscosity results in a substantial increase in the heat transfer as compared to the values found for water. When both top and bottom walls are heated, the local heat transfer enhancement for the mineral oil is smaller as compared with that when each wall is heated alone since the velocity profile and the stress distribution associated with the variable viscosity are more symmetric. Thus the secondary flows are limited to those associated with natural convection and the oil results are in good agreement with the values found in water.

The Evaluation Of A New Model To Simulate Heat Transfer Enhancement In Ribbed Channels

The improvement of overall performance in power to mass ratio of modern gas turbines can greatly be contributed to the effective cooling of the turbine blades. Optimization of coolant performance is essential as the coolant flow introduces losses which need to be minimized. This paper introduces a new model to simulate heat transfer in ribbed channels. In the ribbed sections large variations in turbulence levels occure and the secondary flows associated with these ribs are responsible for significant local heat transfer variations. Furthermore, the ribs produce a complex flow structure so that there is continuing need for a better understanding of the flow physics in ribbed ducts. A finite difference simulation of the fluid flow and temperature distribution in the inlet section of the cooling channel of a typically cooled turbine blade is used to evaluate the new model. Various configurations of rib turbulators in the blade passages were simulated. In the case of the 45° angled ribs, there is a strong sideways secondary flow component which produces both increased overall heat transfer as well as asymmetrical local heat transfer enhancement. The results show the flow structure and heat transfer enhancement and correlate local heat transfer variations to secondary flow details. In general the predicted heat transfer rates compare well with measured data for 90° ribs, but further work needs to be done to accurately model angled ribs. NOMENCLATURE

The Numerical Modelling Of Heat Transfer InTurbine Blades

Advanced aircraft engines employ turbine entry temperatures so high that cooling of the turbine blades is crucial. This paper focuses on the ribs generally used to enhance the heat transfer in turbine blade cooling passages. Secondary flows associated with these ribs are responsible for significant local heat transfer variations. Furthermore, the ribs produce a complex flow structure so that there is continuing need for a better understanding of the flow physics in ribbed ducts. The flow in a channel with four consecutive ribs on two opposite walls and a blockage ratio equal to 0.063 were simulated for angles of 90° and 60° to the flow direction and a Reynolds number of 80000. An inlet condition approximating fully developed flow in a smooth duct were used. In the case of the 60° angled ribs, there is a strong sideways secondary flow component which produces both increased overall heat transfer as well as asymmetrical local heat transfer enhancement. The results show the pressure loss and heat transfer enhancement for both rib angles. In general the predicted heat transfer rates and pressure losses compared well with measured data in the stable area. NOMENCLATURE C^Cj Turbulence constants Ĉ E Turbulence constants k Turbulent kinetic energy p Pressure Pe Peclet number Pr Prandtl number q Heat flux Re Reynolds number St Stanton number v Velocity Vt Tangential velocity d Distance to the wall e Rate of dissipation p Density <7,,cTk Turbulence constants T Shear stress /A Viscosity INTRODUCTION Increased engine efficiency in modern aircraft engines requires a high turbine inlet temperature for a given pressure ratio. These high turbine inlet temperatures are only possible by proper cooling of the different turbine components. Of the components the rotor blades deserve special attention because they have to content with high Transactions on Engineering Sciences vol 5, © 1994 WIT Press, www.witpress.com, ISSN 1743-3533