Asymmetric Heat Transfer Research Articles

304 平行平板間流路内の非対称流れの熱伝達(熱工学II)

304 平行平板間流路内の非対称流れの熱伝達(熱工学II)

Using DTA to quantitatively determine enthalpy change over a wide temperature range by the “mass-difference baseline method”

In thermal analysis, the apparatus commonly used to quantitatively measure enthalpy change is the DSC while the DTA equipment is used for qualitative measurements. In this work a new method, called the “mass-difference baseline method”, is proposed to attempt to use DTA for quantitative measurement. The presented method employs the DTA curve derived from a small mass sample as the baseline for a large mass sample using the same material. Such an approach diminishes the asymmetric heat transfer problem attributed to the “apparatus effect” and “sample influence”, thus greatly improving the linearity between the DTA curve and enthalpy change. The theoretical basis of this method is presented and discussed in this paper. The method has been tested for the determination of the enthalpy change of graphite and elastomers over a large temperature range from 30°C to 600°C. Compared with DSC, one important advantage of this method is that it allows enthalpy measurement to be carried out in an open system allowing a sample mass loss during the measurement. For example, during the measurement of elastomers, intensive decomposition and 70% of the sample mass loss occur. The DTA quantitative measurement successfully determines the heat capacity changes as well as the heat of reaction and heat of evaporation, which all occur during the enthalpy changes of the material subjected to decomposition. However, the accuracy of this method is still not high enough for high accurate enthalpy measurement.

Thermn explosion in conditions of asymmetric heat transfer at the boundaries of plane vessel

Thermn explosion in conditions of asymmetric heat transfer at the boundaries of plane vessel

A Rotating Facility to Study Heat Transfer in the Cooling Passages of Turbine Rotor Blades

This paper describes a new research facility designed to study the effect of rotation on heat transfer in the cooling channels of gas turbine rotor blades. Rotation influences cooling performance via secondary flows generated because of Coriolis forces and centripetal buoyancy. The resulting complex three-dimensional flow creates asymmetric heat transfer over the channel surface. The research facility has been designed to permit experiments to be undertaken that are near to actual engine conditions. The paper includes details of the design philosophy, construction and commissioning of the facility, together with a selection of experimental data.

ANALYSIS OF THERMAL STRESSES IN A FLAT PLATE SUBJECTED TO CONDUCTIVE HEAT TRANSFER WITH THERMAL BOUNDARY CONDUCTANCE

An analysis is presented of the effect of a finite boundary conductance on the magnitude of the thermal stresses in a flat plate subjected to symmetric and asymmetric conductive heat transfer from an infinite, mechanically noninteracting medium.

Mass transfer in turbulent flow

The local mass transfer coefficient in turbulent duct flow was measured using a new experimental technique. The experimental results agreed quite well with a numerical solution to the turbulent diffusion equation in channel flow. Formulation of the diffusion equation provided a continuous solution from a point near the discontinuity in mass flux to the fully developed region. The Nusselt numbers obtained numerically agreed with Spalding's asymptotic solution at x D h < 0·02 and Hatton et al.' seigenvalue solution for symmetrical heat transfer which is valid for x D h > 1 . Current asymmetric heat transfer results in the fully developed region fell between the eigenvalue and numerical solutions.

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

Asymmetric heat transfer in turbulent flow between parallel plates

It would appear from the foregoing that the same is also true for annuli. velocity distribution in smooth pipes. Z. Anger. Mak Mech. 31, 308 (1951). From this work it is apparent that the mixing length 3. constant is not in fact a universal constant for all surfaces. as postulated by some workers, but is in fact a function of the radius or curvature ratio of the annulus to which it applies. 4. The method contained herein appears to give a simple. but reliable, prediction of the form of the u+ yi relationship for the inner region of annuli.

An analytical and experimental study of turbulent gas flow between two smooth parallel walls with unequal heat fluxes

This paper contains the results of a theoretical and experimental study of asymmetric heat transfer to air in fully developed turbulent flow between two smooth parallel plates. The heat fluxes at the plate surfaces were of different magnitude; in the theoretical analysis, each surface heat flux was assumed to be uniform in the flow direction. In the experimental work, two-dimensional flow was simulated by means of long ducts of large aspect ratio. Velocity distribution and friction data in adiabatic flow were recorded, and these showed good agreement with accepted relationships. Heat-transfer measurements were made firstly with one wall insulated and then with heat transfer at both walls. In the latter series of experiments, the wall fluxes were unequal and of opposite sign. The experimental heat-transfer coefficient for the wall through which heat was transferred to the fluid was observed to be less than the accepted value for the case of symmetrical heat transfer and to decrease as the degree of asymmetry increases. A decrease in heat transfer coefficient of up to about 40 per cent was measured. The theory, which is based on the analogy between the transfers of heat and momentum, is more rigorous than that published by the author in a previous paper and is adequately supported by the experimental data. It is concluded that the heat-transfer coefficient is dependent on the heat-flux distribution around the circumference of the flow section.

Convection heat transfer coefficients for turbulent flow between parallel plates with unequal heat fluxes

This paper contains a semi-theoretical analysis of asymmetric heat transfer in fully developed two dimensional turbulent flow between parallel walls. An extension of the analogy between the transfer of heat and the transfer of momentum (due to Mizushina), is used to determine the heat transfer coefficients for the respective boundaries. The results of the analysis are presented in the form of working formulae from which the relative magnitudes of the heat transfer coefficients may be determined. The formulae may be applied to practical engineering problems, but care is necessary because the assumptions made in the theory impose restrictions on the Prandtl and Reynolds number ranges for which the predictions are acceptable. It is immediately obvious from the graphical presentation of the results for a particular fluid, that the heat flux ratio is an important additional parameter, and that cognizance of this should be taken in cases of non-uniform heating. By its nature, the present analysis is preliminary, and an experimental programme has been devised to study the general problem of asymmetric heat transfer and to test the theory. While channel flow has been considered, the results are thought to be valid for axisymmetric flow in annuli with small outside diameter/inside diameter ratios.

A Semi-Theoretical Solution of Asymmetric Heat-Transfer in Annular Flow

A brief account is given of the analogy between the transfer of heat and the transfer of momentum. For asymmetric heat transfer in annular flow, the distributions of friction and thermal flux associated with the transport processes are considered. By simplifying the equations describing these distributions, and using an observed experimental fact concerning the temperature profile, the analogy is modified to apply to the annular flow with a fluid having a Prandtl number equal to 1. The modification results in additional factors in the final form of the conventional heat-transfer equation. These factors are functions of the annulus radius ratio parameter. A comparison is made between the final result and the recommendation of a well known authority.

Asymmetric Heat Transfer to Gases in Turbulent Flow Between Parallel Plates

The simplest case of asymmetric heating of a fluid flowing between parallel plates is considered. This occurs when heat is transferred to the fluid through one plate, the other being adiathermal. For a theoretical analysis, it is convenient to assume fully developed flow in a parallel channel of infinite aspect ratio, this condition being not widely different to that met with in practical applications.