Effect Of Wall Conduction Research Articles

EFFECT OF LONGITUDINAL SEPARATOR SHEET CONDUCTION ON THE TRANSIENT THERMAL RESPONSE OF CROSSFLOW HEAT EXCHANGERS WITH NEITHER GAS MIXED

ABSTRACT The present study investigates the transient temperature behavior of a direct transfer type, single-pass, crossflow heat exchanger with neither gas mixed, accounting for the two-dimensional longitudinal conduction effect on the separator sheet. Numerical results show the variation of the mean exit temperatures of hot and cold gases. The approaching time to steady state is also presented to describe the influence of longitudinal conduction and number of transfer units. Finally, comparisons among step, ramp, and exponential responses are demonstrated to display thermal performance deterioration due to the wall conduction effect.

Effect of wall conduction on melting in an enclosure heated at constant rate

Effect of wall conduction on melting in an enclosure heated at constant rate

Natural convection in a finite wall rectangular cavity filled with an anisotropic porous medium

A numerical study has been made to analyze the wall conduction effect on the natural convection in an anisotropic fluid-saturated porous medium filled in a rectangular cavity. The governing conservation equations are solved by a SIMPLE algorithm. The numerical results indicate that the anisotropic permeability influences the flow field and heat transfer rate significantly. A critical value of the anisotropic thermal diffusivity ratio may exist such that the Nusselt number reaches a minimum. This critical value decreases with increasing the value of the anisotropic permeability ratio. Moreover, wall conductance effect can lead to large changes in the Nusselt numbers.

Transient conjugated heat transfer in channel flows with convection from the ambient

This study presents a numerical solution of the transient conjugated heat transfer in a channel flow with convection from the ambient. The solution takes wall conduction and heat capacity effects into consideration. The effects of wall-to-fluid conductivity ratio K, the dimensionless wall thickness β, the wall-to-fluid thermal diffusivity ratio A and the outside Nusselt number Nu o on the interfacial heat flux, interfacial and bulk temperatures are discussed in detail. Results show that wall effect plays a significant role in the transient conjugated heat transfer problem. Particularly, the wall-to-fluid thermal diffusivity ratio A has a decisive effect with regard to the speed of propagation of thermal energy from the wall-fluid interface to the outer surface of the channel. In addition, the time required for heal transfer to reach the steady state condition is longer for systems with larger K and β or smaller A and Nu o.

The effect of wall conduction on the performance of regenerative heat exchangers

The effect of heat conduction in the wall on the effectiveness of rotary regenerators is described. In order to document the effect of wall conduction, it is assumed that the thermal conductance of the solid is finite, both parallel and perpendicular to the gas flow. Two parameters introduced for modeling conduction effects are the Biot numbers in the y(Bi y ) and x(Bi x ) directions. Our results show that for certain values of Bi x and Bi y , the effectiveness including wall conduction can deviate substantially from results obtained when wall conduction is neglected.

Effect of wall conductivity on thermal stratification

The growing desire to find alternate energy sources to meet the ever-increasing energy demands of mankind has led to extensive research in the field of solar energy. Energy devices working on solar energy need energy storage subsystems because of the intermittent nature of solar radiation. Thermal energy storage systems which keep the warm and cold water separated by means of gravitational stratification have been found to be attractive in low and medium temperature thermal storage applications due to their simplicity and low cost. Experimental investigations have been carried out on scaled model storage tanks of various materials and wall thickness to study their effect on the stratification. It has been found that degradation of thermoclines is slower for tanks whose walls have a higher thermal resistance.

Effects of Wall Conduction and Darcy Number on Laminar Mixed Convection in a Horizontal Square Porous Channel

This paper investigates the effects of peripheral wall conduction and Darcy number on laminar mixed convection in the fully developed region of a horizontal square porous channel with a uniform heat input. Because of the combined effects on nonaxisymmetric channel configuration, buoyancy-induced secondary flow, and noninfinite wall conduction parameter, the flow and heat transfer characteristics are greatly affected by the peripherally nonuniform wall temperature distribution. A modified Darcy-Forchheimer-Ergun flow model in the porous medium and a finite-difference method are applied. Secondary flow patterns, isotherms, wall temperature distributions, the friction factor, and the Nusselt number are presented for a flow in the Darcian porous medium (Da→0) to a pure fluid flow (Da → ∞) with Kp = 10−4 − 104, Gr = 0 − 1010, and Pr = 0.1, 0.73, 7.2, and 100. Wall temperature distributions and the Nusselt numbers are compared with existing experimental data.

Improvements in the analysis of a TH0mp cavity for precise complex permittivity measurements

AbstractAn analysis of the relationship between complex permittivity and complex resonance frequency is proposed for a cylindrical cavity oscillating in a TM0mp mode. Effects of wall conductivity, coupling loops, and holes for the insertion of dielectric samples are fully taken into account. With dielectric samples of small radii, insertion holes produce the most important corrections. Their effect is modelled through a finite difference solution of the field equations in the neighbourhood of the holes. A computer program is described for the analysis, the use of which is illustrated with results of measurements.

WALL CONDUCTION EFFECT ON THE ONSET OF THERMAL INSTABILITY IN HORIZONTAL RECTANGULAR CHANNELS

The effect of peripheral thermal conduction in the bottom wall on the onset of thermal instability in the thermal entrance region of horizontal, rectangular, bottom-uniform-heated channels was studied by experiment and theoretical simulation. The predicted results of the onset of thermal instability for the cases of uniform peripheral wall heat flux are in agreement with the published experimental data, but the predicted results for the cases of uniform peripheral wall temperature exceed the previous data by about an order of magnitude. The present experimental data reconfirm the present predictions. The effect of increasing the peripheral wall conduction is shown to stabilize the flow.

Heat transfer in laminar flow with wall axial conduction and external convection

Analytical solution is obtained for laminar forced convection inside tubes including wall conduction effects in the axial direction, based on a radially lumped wall temperature model, and accounting for external convection. The ideas in the generalized integral transform technique are extended to accommodate for the resulting more involved boundary condition and accurate numerical results obtained for quantities of practical interest such as bulk fluid temperature, lumped wall temperature, and Nusselt number. The effects of external convection and axial conduction along the wall on these heat transfer quantities are then investigated through consideration of typical values for, respectively, Biot number and a wall-to-fluid conjugation parameter. Convergence characteristics of the present approach are also briefly examined.

Effect of noncondensable gas on the performance of a two-phase closed thermosyphon with wall conduction

A theoretical analysis is made to investigate the noncondensable gas effect on the performance of a two-phase closed thermosyphon with wall conduction effect considered. It is shown that the diffusion length of the noncondensable gas increases under the influence of the radial diffusion and the wall conduction effects and these effects are governed mainly by four parameters. The conditions for minimizing these effects are also given.

Theoretical and experimental study of natural convection pipe flows at high Rayleigh number

A combined theoretical and experimental study is performed to investigate natural convection pipe flows at high Rayleigh number. The wall conduction effects and thermal property variations of the fluid and pipe wall are also considered in the analysis. A low-Reynolds-number K- ε turbulence model is employed to treat the transitional and turbulent flow regime including buoyancy effects. The predicted and measured distributions of wall temperature and Nusselt number are in good agreement. Empirical correlations for the induced flow rate and average Nusselt number are proposed. Results show that the characteristics of natural convection heat transfer in a vertical pipe approach those along a single vertical plate for large Rayleigh number.

Steady conjugate heat transfer in turbulent channel flows

The characteristics of steady conjugate heat transfer in turbulent channel flow which is uniformly heated over a finite length are numerically studied. The influences of wall thickness, wall-to-fluid conductivity ratio, and Prandtl number on the nondimensional heat flux and the evolution of temperature field are discussed in great detail. Particular attention is paid to study the wall conduction effects on low-Reynolds number turbulent forced convection heat transfer. The results show that the effects of wall conduction on turbulent forced convection channel flow are significant, especially for the cases with larger wall thickness and wall-to-fluid conductivity ratio.

Effect of wall conduction on free convection between asymmetrically heated vertical plates: uniform wall heat flux

In this study of the effect of wall conduction on laminar free convection between asymmetrically heated vertical plates, an implicit finite difference scheme is used to solve the governing equations. The governing independent parameters are identified to be Prandtl number ( Pr), Grashof number ( Or), ratio of thermal conductivity of the solid to air (K), wall thickness to channel width ratio ( t/ B), channel height to width ratio ( L/ B) and the asymmetric heating parameter ( γ H ). The effect of wall conduction on free convective flow of air under asymmetrically heated conditions ( γ H = 1.0, 0.5, and 0) is discussed. Calculations are made for K = 1 and 10, t/ B = 0.1 and 0.5, Gr = 10-10 4, and L/ B = 1 and 5. The maximum increase in mass flow rate of air for symmetric heating due to wall conduction is 30%. The maximum decrease in average Nusselt number due to wall conduction is 22%. Wall conduction effects are more significant for low Gr flows than for high Gr flows.

Effect of wall conduction on free convection between asymmetrically heated vertical plates: Uniform wall temperature

Effect of wall conduction on free convection between asymmetrically heated vertical plates: Uniform wall temperature

GREEN'S FUNCTION SOLUTION TO HEAT TRANSFER OF A TRANSPARENT GAS THROUGH A TUBE

A heat transfer analysis of a transparent gas flowing through a circular tube of finite thickness is presented. This study includes the effects of wall conduction, internal radiative exchange, and convective heat transfer. The natural mathematical formulation produces a nonlinear, integrodifferential equation governing the wall temperature and an ordinary differential equation describing the gas temperature. This investigation proposes to convert the original system of equations into an equivalent system of integral equations. The Green's function method permits the conversion of an integrodifferential equation into a pure integral equation. The proposed integral formulation and subsequent computational procedure are shown to be stable and accurate.

Numerical and Analytical Solutions for Two-Dimensional Gas Distribution in Gas-Loaded Heat Pipes

This work presents a two-dimensional axisymmetric diffusion model for the non-condensable gas distribution in gas-loaded heat pipes and thermosyphons. The new model, based on an integral analysis, has major advantages over existing, computationally time consuming, two-dimensional models. It has equal accuracy while using only the computational effort required for the cruder one-dimensional model, and also includes the effects of wall conduction and spatial variation of the condenser heat transfer coefficient. To simplify design calculations further an analytic two-dimensional solution is established, which gives excellent results over a wide range of parameters.

Effect of Wall Conduction on Combined Free and Forced Laminar Convection in Horizontal Tubes

Combined free and forced laminar convection in ducts is encountered in a wide variety of engineering applications, including heat exchangers designed for viscous fluids in chemical processes, solar collectors, food industries, and biomedical engineering. In many engineering applications, the boundary conditions for thermal convection in ducts cannot be considered uniform either for wall temperature or heat flux. In the thermally fully developed flow, the effect of peripheral wall conduction may be introduced by the nonuniform peripheral wall heat flux in noncircular ducts and also introduced simultaneously by the secondary flow in rectangular channels, or by the secondary flow alone in circular tubes. This paper investigates the effect of peripheral wall conduction on combined free and forced laminar convection in hydrodynamically and thermally fully developed flow in horizontal circular tubes with uniform heat input axially. A finite-difference power-law scheme is employed to solve the problem.

Simultaneous axial conduction in the fluid and the pipe wall for forced convective laminar flow with blowing and suction at the wall

Numerical solutions are reported for conjugate heat transfer in a porous pipe having an internal flow with blowing or suction at the inner surface of the pipe and constant heat flux at the outer surface. The effect of the simultaneous axial conduction through the wall and the fluid has been studied for the combined hydrodynamic and thermal entry lengths. The results show that the ratio of the thermal conductivities of the pipe wall to the fluid and the thickness of the pipe wall may become significant factors on the heat transfer when the Peclet number is small, especially for the case when fluid is injected into the pipe. It is also shown that the effect of axial wall conduction for the case of constant heat flux at the outer wall surface can be neglected when the wall thickness is small and the ratio of the conductivities of the wall to the fluid approaches unity.

Forced Convection Heat Transfer in a Finitely Conducting Externally Finned Pipe

A numerical study to determine the influence of axial wall conduction on forced convection heat transfer in an externally finned pipe has been made. The effects of wall conductivity, interfin spacing, and external heat transfer coefficient are examined by comparing the results with the corresponding solutions obtained assuming negligible wall conduction. Results indicate that the axial conduction in the pipe walls has a significant influence on the heat transfer behavior. The bulk temperature or the heat transferred to the fluid is underestimated when wall conduction is ignored. At high wall conductivity values, the wall temperatures and Nusselt numbers exhibit a monotonic variation in the axial direction, with the behavior becoming increasingly nonmonotonic as the wall conductivity value is decreased.