Heat Transfer In Rectangular Duct Research Articles

Enhanced heat transfer in rectangular duct with punched winglets

Abstract Thermal performance of a heat exchanger duct with punched winglets (PWs) mounted on the upper duct wall has been examined for Reynolds number (Re) ranging from 4100 to 25,500. In the present experiment, two types of PWs: punched delta- and elliptical-winglets (P-DW and P-EW) with four punched-hole sizes were tested at a fixed attack angle, optimal relative pitch and height. Also, data of solid delta- and elliptical-winglets (DW and EW) were included for comparison. The investigation has shown that the P-DW yields higher thermal-performance enhancement factor (η) than the P-EW. Although the solid DW and EW with no punch have the highest heat transfer and friction loss, the PWs yield better η than the solid ones. For PWs, the P-DW with smaller hole size has the peak heat transfer and friction loss around 5.7 and 40 times over the smooth duct, respectively but the optimum η of 2.17 is seen for the one with a certain hole size. The PWs provide η at about 5%–8% above the solid winglets.

Numerical Investigation of the MHD Flow Under the Influence of Heat Transfer in Rectangular Ducts

Liquid metal (LM) breeder blanket is a topic of great interest in the fusion reactor blanket design because of many advantages. In the breeding blanket of the fusion reactor, the dynamic viscosity of LM is impacted by heat transfer, resulting in the change of velocity distribution. The influence of heat transfer on magnetohydrodynamic flow in a rectangular duct at high Hartmann number is investigated by a coupling method. By this method, the velocity field is calculated through a second-order projection method, coupled with the temperature distribution computed by a finite-volume method. The numerical result without temperature influence is validated by Hunt’s and Shercliff’s analytical solutions, which shows very good accuracy. On the basis of this numerical code, the velocity distribution of a Hunt’s case with temperature influence is simulated. The simulation result indicates that the velocity field is different from the benchmark solution as the result of the heat transfer.

Entropy generation and thermodynamic analysis of solar air heaters with artificial roughness on absorber plate

AbstractThis paper presents mathematical modelling and numerical analysis to evaluate entropy generation analysis (EGA) by considering pressure drop and second law efficiency based on thermodynamics for forced convection heat transfer in rectangular duct of a solar air heater with wire as artificial roughness in the form of arc shape geometry on the absorber plate. The investigation includes evaluations of entropy generation, entropy generation number, Bejan number and irreversibilities of roughened as well as smooth absorber plate solar air heaters to compare the relative performances. Furthermore, effects of various roughness parameters and operating parameters on entropy generation have also been investigated. Entropy generation and irreversibilities (exergy destroyed) has its minimum value at relative roughness height of 0.0422 and relative angle of attack of 0.33, which leads to the maximum exergetic efficiency. Entropy generation and exergy based analyses can be adopted for the evaluation of the overall performance of solar air heaters.

On the Nusselt Number for H2 Heat Transfer in Rectangular Ducts of Large Aspect Ratios

The H1 and H2 forced convection heat transfer in rectangular ducts are studied using an accurate, analytic method. It is confirmed that, as the aspect ratio tends to infinity, the Nusselt number for the H2 case approaches 2.9162, much lower than the parallel plate value of 8.2353 attained by the H1 case. The controversy about the H2 limit is thus settled. An explanation of the behavior is suggested.

The influence of streamwise vortices on turbulent heat transfer in rectangular ducts with various aspect ratios

The effect of aspect ratio of rectangular duct on the turbulent flow and heat transfer is very important for its engineering applications. But the turbulent thermal fields have not been fundamentally scrutinized in spite of its engineering significance especially for cooling device. Hence, in the present study, large eddy simulation is applied to the turbulent flow and heat transfer in rectangular ducts with varying aspect ratio. The turbulent statistics of the flow and thermal quantities are calculated and the characteristics of wall Nusselt number are investigated for each rectangular duct. Especially, to scrutinize near-wall streamwise vortices, a conditional sampling technique is developed and adopted. Clockwise and counter-clockwise rotating streamwise vortices are sampled and the probability density function of the vortex circulation Reynolds number and wall Nusselt number are calculated. From the results, the time-averaged secondary flow caused by instantaneous vortical motions has a great effect on the heat and momentum transport of the flow in the rectangular ducts. Hence, the wall Nusselt number is enhanced near the downwash flow region of the secondary flow. However, with increasing the aspect ratio, the effects of the hot-sweep flow of the clockwise and counter-clockwise rotating vortices become equally dominant near the wall normal bisector of the ducts. During time averaging process, these two counter-rotating vortices are canceled out each other diminishing a secondary flow but they still enhance the wall heat transfer.

An exact analytical solution for convective heat transfer in rectangular ducts

An exact analytical solution is obtained for convective heat transfer in straight ducts with rectangular cross-sections for the first time. This solution is valid for both H1 and H2 boundary conditions, which are related to fully developed convective heat transfer under constant heat flux at the duct walls. The separation of variables method and various other mathematical techniques are used to find the closed form of the temperature distribution. The local and mean Nusselt numbers are also obtained as functions of the aspect ratio. A new physical constraint is presented to solve the Neumann problem in non-dimensional analysis for the H2 boundary conditions. This is one of the major innovations of the current study. The analytical results indicate a singularity occurs at a critical aspect ratio of 2.4912 when calculating the local and mean Nusselt numbers.

Heat transfer enhancement in solar air heater

This paper presents results of an experimental investigation of heat transfer in rectangular duct with repeated ribs in Wcontinuous pattern. The ribs in W-pattern have been tested for both pointing upstream (W-up) and downstream (Wdown) to the flow. The duct has width to height ratio of 8, relative roughness pitch of 10, relative roughness height of 0.03375, angle of attack of ribs in W-pattern is 45° and Reynolds number ranges from 2300-14000. Roughened wall of the duct is uniformly heated while the remaining three walls are insulated. These conditions correspond closely to those found in solar air heaters. The heat transfer results have been compared with those for smooth ducts under similar flow and thermal boundary condition. Enhancement of heat transfer by providing W-shape roughness is 2.39 times to that of smooth plate for W-down ribs and 2.21 times to that of smooth plate for W-up ribs respectively.

Entrance heat transfer in rectangular ducts with constant axial energy input

The study of heat transfer in the entrance region of ducts with different cross-sections is important in engineering practice. This paper considers laminar, hydrodynamically fully developed flow in the thermal entrance regions of rectangular passages, emphasizing heat transfer aspects. By having a prescribed heating or cooling rate and considering the wall temperature to depend on the axial coordinate alone, the temperature solution leads to an integral equation. Solution of this equation is found using an inverse technique to determine the temperature at the walls. For verification purposes, an asymptotic solution is developed which produces results that agree very well with those from the inverse analysis. The results include a correlation and computed values of the Nusselt number at entrance locations, for rectangular ducts with different aspect ratios.

Combined mixed convection and radiation heat transfer in rectangular ducts rotating about a parallel axis

The study of combined heat transfer of convection and radiation in rectangular ducts rotating in a parallel mode was investigated numerically in detail. The coupled momentum and energy equations are solved by the DuFort–Frankel numerical scheme to examine the interactions of convection with radiation. The integro-differential radiative transfer equation is solved by the discrete ordinates method. Results are presented over a wide range of the governing parameters. The present results reveal that the rotational effect in a square duct is more significant than that in a rectangular one. The predictions also demonstrate that the radiation presents significant effects on the axial distributions of the total Nusselt number, Nu t, and tends to reduce the centrifugal-buoyancy effects. The effect of rotation on the Nu t is restricted in the entrance region, however, the radiation affects the heat transfer through out the channel. Additionally, the Nu t increases with the decrease in the conduction-to-radiation parameter N C.

The effects of imperfect insulator coatings on MHD and heat transfer in rectangular ducts

The effects of imperfect insulator coatings on MHD and heat transfer in rectangular ducts

The effects of imperfect insulator coatings on MHD and heat transfer in rectangular ducts

Abstract The integrity of the electrically insulating coating at the coolant channel walls represents a feasibility issue for a self-cooled liquid metal blanket. The effects of cracks in the insulating layer on MHD pressure drop are investigated, for various crack locations, sizes and resistivities. It is shown that, once the crack resistance exceeds the Hartmann layer resistance, the MHD pressure drop increases significantly. Using the 2-D fully-developed MHD flow code, it has been found that the crack location has a large impact on the MHD pressure drop: for the same crack sizes and resistivities, the pressure gradient increases as the cracks move towards the central plane. The effect of the insulation on heat transfer is discussed. The heat transfer capability is reduced in the presence of the insulator coating.

Non-Newtonian fluid laminar flow and forced convection heat transfer in rectangular ducts

Numerical solutions for fully developed laminar flow forced convection heat transfer of a power law non-Newtonian fluid in rectangular ducts are presented. Finite difference methods are developed for the governing equations to obtain the velocity and temperature distributions. Friction factor results are given for flow through rectangular ducts of aspect ratios of 0.2, 0.5 and 1.0 with power law index n values of 0.5 to 1.0. For the same flow conditions the Nusselt values, maximum wall temperatures, and minimum wall temperatures for the H2 thermal boundary condition for different combinations of heated and adiabatic walls are obtained. Also the Nusselt values for slug flow ( n=0 ) are presented for the H2 boundary condition.

Laminar forced convection heat transfer in rectangular ducts rotating about an axis parallel to duct axis.

Fully developed steady laminar flow of viscous fluid in straight rectangular ducts rotating about a parallel axis is analyzed numerically. Thermal boundary condition of axially uniform wall heat flux is considered. Numerical results of flow and temperature field in ducts with aspect ratio from 0.2 to 5 are obtained for ReRaΩ number upto 6.725×106 (for Re numbers upto 1346) and Prandtl number from 0.3 to 400. At low to moderate ReRa numbers, a double-vortex secondary flow is exist under the influence of buoyancy due to centrifugal force field. It is found that a pair of additional vortices appear and disappear for higher range of ReRaΩ numbers. It is shown that tritical number of ReRaΩ for onset of additional vortices is affected by Prandtl number and aspect ratio. The friction factor and Nusselt number are pressented. The solution is compared with other analyses.

Turbulent heat transfer in rectangular ducts with repeated-baffle blockages

An experimental study is conducted to investigate heat transfer and pressure drop in a rectangular duct with repeated-baffle blockages. The baffles are arranged in a staggered fashion with fixed axial spacing. The transfer coefficients are evaluated in the periodic fully developed and entrance regions of the duct. The presence of the baffles enhances these coefficients. The entrance length of the duct is substantially reduced by the baffles. Finally, pressure drop and heat transfer data are employed to evaluate the thermal performance of the duct.

Enhancement of convective heat transfer in rectangular ducts of interrupted surfaces

Results are presented from systematic experimental investigations into the enhancement of heat transfer of 11 interrupted surfaces of one type and of different size formed by a multitude of short rectangular ducts with a constant cross-sectional aspect ratio of 7:1 in the absence of burrs and bends on the sharp edges of fins. Investigations were carried out with air in the range of Reynolds numbers from 500 to 10000 for geometric parameters l'/ d = 0.65−3.24 and δ = 0.058−0.114. Based on these investigations, standard experimental thermohydraulic characteristics of these surfaces are obtained as well as their correlations as functions of Re and geometric parameters l'/ d and δ/ d. New results are obtained on the effect of the geometric parameters l'/ d and δ/ d of interrupted surfaces on the enhancement of heat transfer in them and also on the experimental determination of the coefficients of overall pressure drop at the entrance and exit from the interrupted surface in a heat exchanger.

Laminar forced convection heat transfer in rectangular ducts rotating about an axis perpendicular to the duct axis. (2nd report Numerical analysis in case of rectangular ducts with aspect rations 0.2, 0.5, 2 and 5)

Numerical analysis is conduced on the forced convection heat transfer in rectangular ducts with aspect ratios 0.2, 0.5, 2.0 and 5, 0, rotating at a constant angular velocity about an axis perpendicular to its axis. Navier-Stokes and energy equations are solved by a finite-difference method in the fully developed region of laminar flow. Velocity and temperature fields are obtained for various rotation rates and Reynolds numbers at each aspect ratio of the duct. A double-vortex secondary flow appears at low to moderate rotation rates. An additional pair of vortices appeared and disappeared for a higher range of Reynolds numbers due to aspect ratios of the ducts. The friction factor and Nusselt number are influenced by the Coriolis force and aspect ratio. Numerical results of the friction factor for aspect ratios equal to 0.5 and 2.0 are in good agreement with experimental results by other researchers.

Laminar forced convection heat transfer in rectangular ducts rotating about an axis perpendicular to duct axis. (1st report Numerical analysis in case of square ducts)

Fully developed steady laminar flow of viscous incompressible fluid in straight square ducts rotating at a constant angular velocity about an axis perpendicular to its axis under thermal conditions of axially uniform wall heat flux and peripherally uniform wall temperature at any axial position is studied. Numerical solutions were obtained for ReRΩ numbers from 10 to 105 (for Re numbers form 1 to 4200). At low to moderate ReRΩ numbers, a doublevortex secondary flow appears under the influence of the Coriolis force. For higher ReRΩ numbers it is found that an additional counter-rotating pair of vortices appeared and disappeared for the range of RΩ numbers from 10 to 100. Correlation equations for friction factors and Nusselt numbers are obtained from numerical results. The numerical results in regard to the friction factor and Nusselt number are in good agreement with experimental and analytical results by other reseachers for circular pipe.

Combined convection and radiation in rectangular ducts

Combined convective and radiative heat transfer in rectangular ducts is analyzed by solving simultaneously the fluid dynamic equations and the radiation transport equation. The major approximations invoked in the analysis are the assumption of parabolic flow for the fluid equations and the use of the moment method for simplifying the radiation transport equation. The analysis is applicable to general compressible, turbulent, developing, radiating flow in rectangular ducts in the presence of scattering particles. In the sample calculations, Mie theory is used for determining the spectral absorption and scattering coefficients of the particles.

Developing Heat Transfer in Rectangular Ducts With Staggered Arrays of Short Pin Fins

Streamwise heat transfer variation, overall array heat transfer, and overall flow friction behavior are presented for large aspect ratio ducts containing uniformly spaced staggered arrays of circular pin fins which span the entire duct height. The array geometries investigated have short pin heights (length-to-diameter ratio of 1) and moderate spanwise (transverse) and streamwise (longitudinal) pin spacings (pitch-to-diameter ratios of 1.5 and 2.5). Such staggered array geometries are typical of applications found in internally cooled gas turbine engine airfoils. The uncovered duct walls comprise a substantial fraction of the total heat transfer area. Ten pin rows in the streamwise direction are utilized in all the experiments, with a segmented construction allowing determination of spanwise-averaged heat transfer coefficients resolved to a single row spacing in the streamwise direction. Comparisons are made with results of prior studies which are mainly restricted to flow normal to banks of circular cylinders with long length-to-diameter ratios.

Forced convection heat transfer in rectangular ducts—general case of wall resistances and peripheral conduction for ventilation cooling of nuclear waste repositories

A numerical solution for laminar flow heat transfer between a flowing gas and its containing rectangular duct has been obtained for many different boundary conditions which may arise in nuclear waste repository ventilation corridors. The problem has been solved for the cases of insulation on no walls, one wall, two walls, and three walls with various finite resistances on the remaining walls. Simplifications are made to decouple the convective heat transfer problem from the far field conduction problem, but peripheral conduction is retained. Results have been obtained for several duct aspect ratios in the thermal entrance and in the fully developed regions, including the constant temperature cases. When one wall is insulated and the other three are at constant temperature, the maximum temperature occurs in the fluid rather than on the insulated wall. This maximum moves toward the insulated wall with increasing axial distance. Nusselt numbers for the same constant flux on all four walls with peripheral conduction lie in a narrow band bounded by zero and infinite peripheral conduction cases. A dimensionless wall conduction group of four can be considered infinite for the purpose of estimating fully developed Nusselt numbers to within an accuracy of 3%. A decrease in wall and bulk temperatures by finite wall conduction has been demonstrated for the case of a black body radiation boundary condition. Nusselt numbers for the case of constant temperature on the top and bottom walls and constant heat flux on the side walls exhibited unexpected behavior.