Heat Transfer In Duct Research Articles

Simulation of air pipe heat dissipation in civil aircraft water system based on flowmaster

Abstract The water supply method of civil aircraft water system is mainly from the bleed air system to pressurize the water tank. However, the gas temperature from the bleed air system is often too high. The AC-25.863 draft [1] stipulates that the maximum surface temperature of the equipment should be lower than 50°F below the spontaneous combustion point of flammable liquid in the area. Otherwise, it will become the ignition source of flammable materials. In this paper, by establishing the heat transfer model of an air pipeline, a set of air duct heat transfer calculation methods is proposed, and the variation rule of air pipe surface temperature is studied, to verify that the gas temperature from the bleed air system meets the requirements of the system, and the air pipe surface temperature meets the requirements of AC-25.863 draft.

Radiation-convective heat transfer and performance analysis of a parallel-plates duct direct absorption solar heat collection system

Solar photo-thermal conversion and solar heat collection are hot topics today. However, which solar collection mode is the best? The question has puzzled scholars for many years. In this study, a direct absorption solar heat collection (DASC) duct system was investigated. Carbon quantum dots (CQDs) were selected as the heat absorption medium. Different absorption modes were clarified. It is found that the ratio of the radiation penetration depth of nanofluids to duct depth (duct penetration ratio, α) plays a key role in the duct heat transfer and system performance. Totally three solar heat collection modes can be classified depending on the value of α: volume absorption (partial volume 0<α<1 or whole volume α = 1), surface absorption (upper α → 0 or bottom α → ∞) and hybrid absorption for α > 1. Of them, the whole volume absorption has the best performance when α = 1, while the hybrid performs the best when the penetration ratio is not unity. Surprisingly, a higher fluid absorption coefficient does not necessarily lead to a higher photo-thermal performance. The extreme high fluid absorption coefficients (upper surface mode) are inferior to extreme low absorption coefficients (bottom surface mode). Of the different modes, the total Nusselt numbers in the whole volume absorption mode are 2–3 times higher than those in the bottom surface absorption mode. For solar heat collection, convective heat collection is better than standstill collection, and a certain flow velocity should be ensured to have a satisfactory efficiency. The findings were experimentally verified.

Physically Realistic Inlet-Velocity Profiles in Duct Flows

A uniform inlet velocity profile is widely used in the numerical simulations of fluid flow and heat transfer in ducts for both incompressible and compressible flows. In incompressible flows, the calculated fluid pressure at the inlet edge is extremely high and affects the calculation of the average pressure. In compressible flows, the fluctuation of pressure in the flow direction results in the fluctuation in the velocity. This has motivated this study to numerically investigate a physically realistic velocity profile at the inlet of a pipe rather than using a uniform velocity profile. The numerical simulations were based on the control volume-based power law scheme and the semi-implicit method for pressure-linked equations (SIMPLE) algorithm. The continuity and momentum equations for a flow in a pipe with the rounded inlet corner were solved to obtain a physically realistic inlet velocity profile. The obtained inlet velocity profile was expressed by a simple expression in the range of Reynolds number from 100 to 2000. Using this velocity profile, both the incompressible and compressible flows in a pipe were numerically investigated. The results resolved the previously observed inconsistencies in the pressure that were previously observed in the numerical simulations with uniform inlet velocity profiles.

Heat Transfer and Fluid Flow in Concentric Annular Ducts Using the Galerkin-Based Integral Method: A Numerical Study

In this paper, we cope with the problem of presents a numerical analysis for heat transfer in a duct with geometry circular annular elliptical using the Galerkin-based integral method. The analysis is performed for different geometries of the duct (circular annular circular and circular annular elliptical), and the method is validated for circular cylindrical geometry. Parameters such as mean temperature and mean and location Nusselt numbers for two boundary conditions: constant wall temperature and axial constant heat flux in the wall with constant wall temperature are presented and analyzed.

Numerical analysis of Al2O3-water nanofluids through square cross-section duct with single phase models

In this study, the effect of various concentrations of Al2O3-water nanofluid on heat transfer in duct with a square cross-section, under conditions of continuous heat flux and laminar forced flow, investigated numerically. Reference experi-mental application parameters are accepted as reference, calculated with single phase homogeneous model (SPHM) and single phase Brownian model (SPBM) models, and the average heat transfer coefficient results of numerical models are compared with each other and reference experimental data. Numerical results obtained with SPHM and SPBM are in comparison to the relevant experimental data. For SPHM and SPBM results, 0.5%, %1.5, %2.5 Al2O3-water nanofluids, the average deviation rate for the average heat transfer coefficient is 3.35% and 2.36%, 5.24%, 3.43%, 7.74%, and 5.3% according to the reference experimental data respectively. 0.5%, %1.5, %2.5, 2.24%, and 3.81%, 5.86% and 7.47%, 9.58% and 11.6%, respectively, the average heat transfer coefficient is increased by using Al2O3-water nanofluid compared to water. It has been observed that, SPBM has closer results than SPHM model according the relevant experimental data.

Converging-Diverging Duct Heat Transfer and Hydrodynamic Resistance for Turbulent Water Flow.(Dept.M)

Converging-Diverging Duct Heat Transfer and Hydrodynamic Resistance for Turbulent Water Flow.(Dept.M)

New correlations for heat transfer in parallel-plate ducts at low Peclet number

Calculations of heat transfer in duct flows generally neglect the effect of axial heat conduction within the fluid. However, in some situations, this effect must be taken into account. The decision on whether or not to consider this phenomenon is driven by the value of the Peclet number. A low value, with 100 commonly considered the threshold, indicates that axial conduction, i.e., conduction parallel to the flow, should be accounted for in the calculations. Microfluidic devices, systems using liquid metal coolant or microchannel heat exchangers are some examples of applications where low Peclet flow conditions may be encountered. While exact analytical solutions exist for such cases, they are often impractical to use for common engineering design purposes due to their mathematical complexity. The present work proposes three new correlations for the estimation of the fully developed Nusselt number, thermal entry length and local Nusselt number in the entrance region of a laminar flow within a parallel-plate duct with constant wall temperature. These correlations allow the simple and accurate evaluation of useful engineering quantities, thus avoiding lengthy and complex calculations. They were developed using rational approximation theory and are effective on a wide range of Peclet numbers and axial coordinates. Accuracy of under 1% relative error is achieved with each of the three correlations when results are compared against exact analytical series solution values.

Effect of half circular obstacles in duct on natural heat transfer: numerical study

The natural heat transfer in ducts depend on generating an induced flow due to the difference in density between the heated fluid and it is environment. The effect of reduce cross sectional area of a vertical isothermal duct by adding half circular obstacles to the inner side of the duct. The effect of the induced flow and obstacles were study using COMSOL software and compared to heat through same duct with no flow. The results that obstacles increase induced flow by 16% compared to normal vertical duct, Also shows a reduction in value of the maximum temperature of the hot surface at the solar peak heat flux (3:00 PM).

In Memoriam: Professor Ephraim Maurice Sparrow

In Memoriam: Professor Ephraim Maurice Sparrow

Experimental and numerical analysis of heat transfer and flow characteristics in parabolic ducts

In this paper, the heat transfer and flow characteristics of parabolic ducts with different focal lengths (P = 120 mm, 140 mm, and 160 mm) are analyzed and compared with rectangular and trapezoidal ducts respectively. The governing equations in the curved ducts, including the mass conservation equation, momentum equation and energy conservation equation, are established. The results show that the order of the overall heat transfer performance of the three ducts from the best to worst is parabolic duct, trapezoidal duct and rectangular duct. The parabolic duct has the best overall enhanced heat transfer performance, which is 5.1% and 25.4% larger than that of the trapezoidal and the rectangular ducts respectively. This means that, although the divergent type structure reduces the flow velocity and thus weakens the intensity of convective heat transfer in the duct, it improves the downstream heat transfer area and reduces the pressure drop loss in the flow process, thus promoting the improvement of overall heat transfer performance. Besides, the temperature difference in the parabolic flow duct has the most uniform distribution, which is an important factor to improve and strengthen heat transfer performance. From the perspective of field synergy theory, the average field synergy angle decreases with the increase of focal length, and the field synergy becomes better.

CONVECTION HEAT TRANSFER IN A CHANNEL OF DIFFERENT CROSS SECTION FILLED WITH POROUS MEDIA

A forced convection heat transfer in ducts (circular, triangular, rectangular) cross sections and (1m) length with hydraulic diameter (0.1m) filled with porous media (glass spheres 12 mm diameter) is investigated experimentally at constant heat flux from the wall (1070W/m²) with Reynolds number range of (12461-2500). Comparison was made between three ducts for local temperature distribution and local Nusselt number). The experimental results showed the effect of Reynolds number and cross section on the temperature profile and local Nusselt number,also empirical correlations for average Nusselt number and Peclet number were obtained for three ducts.

Axial heat conduction effects in the thermal entrance region for flows in concentric annular ducts: Correlations for the local bulk-temperature and the Nusselt number at the outer wall

Streamwise heat conduction in the flow can affect the heat transfer in ducts. This is mainly relevant for flows with small Prandtl numbers or in micro-channels. In both situations the Péclet number is small, so axial heat conduction in the fluid cannot be neglected. In literature analytical solutions of this so called extended Graetz problem are well documented. In the present paper correlations are developed for the local bulk-temperature and the local Nusselt number at the outer wall in the thermal entrance region of concentric annular ducts. Correlations are given for laminar and turbulent fully-developed flow and different thermal boundary conditions. The correlations also consider the effect of different ratios of the inner radius to the outer radius of the annular ducts.

Slip Flow and Constant Flux Heat Transfer in Isosceles Triangular Ducts

The Ritz method for slip flow and constant flux heat transfer in ducts of general shape is established. The efficient Ritz method is applied to the isosceles triangular duct, yielding useful tables for the Poiseuille number and the Nusselt numbers. The results are generic, such that they can be applied to both rarefied gas flows and superhydrophobic flows.

Passive techniques for the enhancement of convective heat transfer in single phase duct flow

This review presents the main results of the experimental campaign on passive techniques for the enhancement of forced convective single phase heat transfer in ducts, performed in the last years at the Laboratory of the Industrial Engineering Department of the University of Parma by the Applied Physics research group. The research was mainly focused on two passive techniques, widely adopted for the thermal processing of medium and high viscosity fluids, based on wall corrugation and on wall curvature. The innovative compound heat transfer enhancement technique that couples together the effect of wall curvature and of wall corrugation has been investigated as well. The research has been mainly focused on understanding the causal relationship between the heat transfer surface modification and the convection enhancement phenomenon, by accounting the effect of the fluid Prandtl number. The pressure loss penalties were also evaluated. The principal results are presented and discussed.

An Investigation Into the Effects of Highly Transient Flight Maneuvers With Heat and Mass Transfer on the T-38 Air Force Trainer Inlet

The T-38 talon currently serves as the primary United States Air Force trainer for fighter aircraft. This supersonic trainer was developed in the 1960s but continues to be used today as the result of various modernization programs throughout its service life. The latest propulsion modernization program focused on improved takeoff performance of the T-38’s inlets, improved reliability of the twin J85 afterburning turbojet engines, and reduced drag with an improved exhaust nozzle design. The T-38’s inlet includes bleed holes upstream of the engine face to provide cooling airflow from the inlet to the engine bay. However, at various flight conditions, the bay air is pressurized relative to the inlet, resulting in reverse flow of hot engine bay air into the inlet. This reverse flow causes total temperature distortion that may reduce the engine stability margin. Partial inlet instrumentation of the left engine was used to estimate the total temperature distortion associated with reverse flow, however, flight testing of highly transient maneuvers revealed levels of total temperature distortion greater than that predicted for reverse flow alone. This discovery led to the hypothesis that thermal energy storage of the aluminum inlet during transient flight maneuvers resulted in increased temperature distortion at the engine face. Flight data analysis demonstrated the need for a near-real-time thermal inlet distortion analysis capability. A two-dimensional (2D) transient axisymmetric heat and mass transfer model was developed through the use of a lumped-parameter boundary-layer model to simulate the inlet flow and determine the time-dependent inlet duct heat transfer. This model was validated with transient 2D computational fluid dynamics and two flight maneuvers. The analysis of flight maneuvers revealed that in the absence of engine bay air re-ingestion, the time lag associated with the heating and cooling of the inlet walls generates radial temperature distortion, which has the effect of reducing engine stability margin up to 5.44% for the maneuvers analyzed.

Determination of Heat Transfer in Ducts With Axial Conduction by Variational Calculus

This study uses a methodology based on the calculus of variation to determine the heat transfer in passages with two-dimensional velocity fields such as rectangular channels and in the presence of axial conduction. The mathematical procedure is presented and the subsequent numerical computations provide the Nusselt number values. To verify the accuracy of this numerical procedure, the Nusselt number values are acquired for parallel-plate channels and circular pipes and compared with similar data from the Graetz-type exact analyses. Then, rectangular passages are selected to show the capability and a square duct is used to study the domain of accuracy for this procedure. The results for small Peclet numbers lead to a simple correlation for determination of the bulk temperature and they compare well with those obtained from an asymptotic solution.

Temperature field in a moving semi-infinite region with a prescribed wall heat flux

Steady state conduction of heat from a stationary wall to a medium moving at a uniform velocity is the subject herein. This medium can be a solid or a fluid moving at a constant velocity. The surface of this medium is insulated until a change in the surface heat flux occurs. The determination of temperature field is the main objective herein. The results show that the surface temperature begins to increase before its arrival to the heater’s location where there is an abrupt change in the surface heat flux. The application of this phenomenon to a moving wall with frictional heating at its surface and to classical heat transfer in ducts can lead to new information.

Heat transfer characteristics of water flow in microtubes

This study provides an experimental investigation on forced convective heat transfer performance of water flowing through six microtubes with inner diameters ranging from 123 to 962 μm. A non-contacted liquid crystal thermography (LCT) temperature measurement method that proposed by Lin and Yang [T.-Y. Lin, C.-Y. Yang, An experimental investigation on forced convection heat transfer performance in microtubes by the method of liquid crystal thermography, International Journal of Heat and Mass Transfer (2007), doi:10.1016/j.ijheatmasstransfer.2007.03.038] was used in this study to measure the surface temperature of microtubes. The test results show that the conventional heat transfer correlations for laminar and turbulent flow can be well applied for predicting the fully developed heat transfer performance in microtubes. The transition occurs at Reynolds number from 2300 to 3000. This is also the same range as that for conventional tubes. There is no significant size effect for water flow in tubes within this diameter range. The laminar thermal entrance length for microtubes is longer than that estimated by the conventional correlation. The developing Nusselt numbers for 962 μm tube agree well with those predicted by the Shah and Bhatti [R.K. Shah, M.S. Bhatti, Laminar convective heat transfer in ducts, in: S. Kakac, R.K. Shah, W. Aung, (Eds.), Handbook of Single-Phase Convective Heat Transfer, Willy, New York, 1987] correlations. However, as the tube size decreases, the discrepancy between the test results and the predicting value increases.

Experimental study and artificial neural network modeling of unsteady laminar forced convection in a rectangular duct

In this study, an artificial neural network (ANNs) for the prediction of unsteady heat transfer in a rectangular duct was studied. The ANNs has been applied for the unsteady heat transfer in a rectangular duct. An experimental study has been carried out to investigate the axial variation of inlet temperature and the impact of inlet frequency on decay indices in the thermal entrance region of a parallel plate channel. The investigation was conducted with laminar forced flows for Reynolds numbers ranging from 1,120 to 2,200 while the inlet heat input frequency varied from 0.02 to 0.24 Hz. The results revealed that the ANNs can be used for modeling unsteady heat transfer in the duct. The accuracy between experimental and ANNs approach results was achieved with a mean absolute relative error less than 39%.

Unsteady RANS simulation of turbulent flow and heat transfer in ribbed coolant passages of different aspect ratios

The flow and heat transfer in ribbed coolant passages of aspect ratios (AR) 1:1, 4:1, and 1:4 are numerically studied through the solution of the unsteady Reynolds averaged Navier–Stokes (URANS) equations. The URANS procedure, which utilizes a two equation k– ε model for the turbulent stresses, is shown to resolve large-scale bulk unsteadiness. The computations are carried out for a fixed Reynolds number of 25,000 and density ratio of 0.13, while the Rotation number is varied between 0.12 and 0.50. At higher rotation numbers (⩾0.5) at least three inter-rib modules are required to ensure periodicity in the streamwise direction. The flow exhibits unsteadiness in the Coriolis-driven secondary flow and in the separated shear layer. The average duct heat transfer is the highest for the 4:1 AR case. For this case, the secondary flow structures consist of multiple roll cells that direct flow both to the trailing and leading surfaces. The 1:4 AR duct shows flow reversal along the leading surface at high rotation numbers. For this AR, the potential for conduction-limited heat transfer along the leading surface is identified. The friction factor reveals an increase with the rotation number, and shows a significant increase at higher rotation numbers (∼ Ro = 0.5).