Constant Wall Temperature Conditions Research Articles

Numerical Investigation of Flow Friction and Heat Transfer in a Channel with Various Shaped Ribs Mounted on Two Opposite Ribbed Walls

Heat transfer and fluid flow characteristics through a channel with two - dimensional transverse ribs are numerically investigated. The effects of different shaped ribs (rectangular rib, triangular rib, cylindrical rib, concave-concave rib, convex-concave rib, long convex-short concave rib and long concave-short concave rib) are examined. The investigation is performed for the ribs mounted on top and bottom walls with rib pitch to height ratio (P/e) of 6.67, in Reynolds number between 3000 and 7000, under constant wall temperature condition. For the present case, the data predicted by SST k-omega turbulence model show better agreement with the experimental data, than those predicted by RNG k-epsilon turbulence model. The mean heat transfer, flow friction, temperature fields and the local heat transfer coefficients as well as the flow structure behaviors are reported. For the ribs considered, the channel with triangular-ribs yields the highest Nusselt number, the one with concave-concave rib provides the highest friction and the one with cylindrical rib show the best thermal enhancement. At the similar condition, the triangular rib gives higher Nusselt number than that provided by the cylindrical one by around 29.3 percent, while the cylindrical rib offers the highest thermal enhancement factor of 1.4 which is higher than that of the cylindrical one by around 1.33 percent while the cylindrical rib offers the highest thermal enhancement factor of 1.33 which is higher than that of the triangular one by around 9.6 percent. In addition, in spite of their high heat transfer, the concave-concave rib and long concave-concave rib yield low thermal performance factors, due to the prominent effect of high friction factor.

Numerical investigation of Al2O3/water nanofluid laminar convective heat transfer through triangular ducts

In this article, laminar flow-forced convective heat transfer of Al2O3/water nanofluid in a triangular duct under constant wall temperature condition is investigated numerically. In this investigation, the effects of parameters, such as nanoparticles diameter, concentration, and Reynolds number on the enhancement of nanofluids heat transfer is studied. Besides, the comparison between nanofluid and pure fluid heat transfer is achieved in this article. Sometimes, because of pressure drop limitations, the need for non-circular ducts arises in many heat transfer applications. The low heat transfer rate of non-circular ducts is one the limitations of these systems, and utilization of nanofluid instead of pure fluid because of its potential to increase heat transfer of system can compensate this problem. In this article, for considering the presence of nanoparticl: es, the dispersion model is used. Numerical results represent an enhancement of heat transfer of fluid associated with changing to the suspension of nanometer-sized particles in the triangular duct. The results of the present model indicate that the nanofluid Nusselt number increases with increasing concentration of nanoparticles and decreasing diameter. Also, the enhancement of the fluid heat transfer becomes better at high Re in laminar flow with the addition of nanoparticles.

Two-phase annular flow and evaporative heat transfer in a microchannel

A physical and mathematical model has been developed to predict the two-phase flow and heat transfer in a microchannel with evaporative heat transfer. Sample solutions to the model were obtained for both constant wall temperature and constant wall heat flux conditions. Results are provided for evaporation rate, liquid film thickness, liquid and vapor phase pressure and temperature distributions. In addition to the sample calculations that were used to illustrate the transport characteristics, computations based on the current model were performed to generate results for comparisons with the experimental results of Qu and Mudawar (2004) where two different mass flow rates of the working fluid were used in the experiment. The comparisons of total pressure drops with the experimental data of Qu and Mudawar (2004) cover the wall heat flux range of 142.71–240 W/cm 2 with a total channel mass flux of 400.1 kg/m 2 s and also the wall heat flu range of 99.54–204.39 W/cm 2 with total channel mass flux of 401.9 kg/m 2 s. The calculated results from the current model match closely with those of Qu and Mudawar (2004).

Evaluation of the heat transfer performance of helical coils of non-circular tubes

This study addresses heat transfer performance of various configurations of coiled non-circular tubes, e.g., in-plane spiral ducts, helical spiral ducts, and conical spiral ducts. The laminar flow of a Newtonian fluid in helical coils made of square cross section tubes is simulated using the computational fluid dynamic approach. The effects of tube Reynolds number, fluid Prandtl number, coil diameter, etc., are quantified and discussed. Both constant wall temperature and constant heat flux conditions are simulated. The effect of in-plane coil versus a cylindrical design of constant coil, as well as a conical coil design is discussed. Results are compared with those for a straight square tube of the same length as that used to form the coils. Advantages and limitations of using coiled tubes are discussed in light of the numerical results.

Enhancement of jet-to-wall heat transfer using axisymmetric grooved impinging plates

Abstract An experimental investigation was carried out to examine the effects of axisymmetric lathe-worked grooves on the impinging jet-to-wall heat transfer, under constant wall temperature conditions. This study covers jet Reynolds numbers, based on the orifice diameter D, from 15 000 to 30 000, for a given jet-to-wall dimensionless distance H/D = 2. The grooves have either square or triangular cross-section, with depth c = 1 mm, and pitch p = 2 mm. Under these conditions, we obtained significant heat transfer enhancements, up to 81% as compared with the smooth plate reference case, for a value of the dimensionless plate radius R/D = 2, a jet Reynolds number Rej = 23 000, and for square cross-section lathe-worked grooves.

Numerical study of convective heat transfer of nanofluids in a circular tube two-phase model versus single-phase model

Laminar convective heat transfer of nanofluids in a circular tube under constant wall temperature condition is studied numerically using a CFD 1 1 Computational Fluid Dynamics. approach. Single-phase and two-phase models have been used for prediction of temperature, flow field, and calculation of heat transfer coefficient. Effects of some important parameters such as nanoparticle sources, nanoparticle volume fraction and nanofluid Peclet number on heat transfer rate have been investigated. The results of CFD simulation based on two-phase model were used for comparison with single-phase model, theoretical models and experimental data. Results have shown that heat transfer coefficient clearly increases with an increase in particle concentration. Also the heat transfer enhancement increases with Peclet number. Two-phase model shows better agreement with experimental measurements. For Cu/Water nanofluid with 0.2% concentration, the average relative error between experimental data and CFD results based on single-phase model was 16% while for two-phase model was 8%. Based on the results of the simulation it was concluded that the two-phase approach gives better predictions for heat transfer rate compared to the single-phase model.

3-D Numerical simulation of swirling flow and convective heat transfer in a circular tube induced by means of loose-fit twisted tapes

The article presents the application of a mathematical model for simulation of the swirling flow in a tube induced by loose-fit twisted tape insertion. Effects of the clearance ratio defined as ratio of clearance between the edge of tape and tube wall to tube diameter (CR = c/ D = 0.0 (tight-fit), 0.1, 0.2 and 0.3) on heat transfer enhancement (Nu), friction factor ( f) and thermal performance factor ( η) are numerically investigated for twisted tapes at two different twist ratios ( y/ w = 2.5 and 5.0). The simulation is conducted in order to gain an understanding of physical behavior of the thermal and fluid flow in the tube fitted with loose-fit twisted tape under constant wall temperature conditions in the turbulent flow regime for the Reynolds number ranging from 3000 to 10,000. The Navier–Stokes equation in common with a energy equation is solved using the SIMPLE technique with the standard k– ε turbulence model, the Renormalized Group (RNG) k– ε turbulence model, the standard k– ω turbulence model, and Shear Stress Transport (SST) k– ω turbulence model. The numerical results show that the predictions of heat transfer (Nu) and friction factor ( f) based on the SST k– ω turbulence models are in better agreement with Manglik and Bergles [R.M. Manglik, A.E. Bergles, Heat transfer and pressure drop correlations for twisted-tape inserts in isothermal tubes, part II: Transition and turbulent flows, Transaction ASME, Journal of Heat Transfer, 115 (1993) 890–896.] than other turbulence models. The mean flow patterns in a tube with loose-fit twisted tapes in terms of contour plots of velocity, pathline, pressure, temperature and turbulent kinetics energy (TKE) are presented and compared with those in a tube fitted with tight-fit twisted tapes. It is visible that the twisted tape inserts for y/ w = 2.5 with CR = 0.0 (tight-fit), 0.1, 0.2 and 0.3 can enhance heat transfer rates up to 73.6%, 46.6%, 17.5% and 20%, respectively and increase friction factors up to 330%, 262%, 189%, and 160%, respectively, in comparison with those of the plain tube. The tube with loose-fit twisted tape inserts with CR = 0.1, 0.2 and 0.3 provide heat transfer enhancement around 15.6%, 33.3% and 31.6% lower than those with CR = 0.0 (the tight-fit twisted tape). The heat transfer augmentation is expected to involve the swirl flow formation between the tape and a tube wall. In addition, the simulation for thermal performance factor ( η) of a tube with the loose-fit twisted tape and the tight-fit twisted tape under the same pumping power is also conducted, for comparison.

The Effect of Double Dispersion on Natural Convection Heat and Mass Transfer in a Non-Newtonian Fluid Saturated Non-Darcy Porous Medium

The effect of power law index parameter of the non-Newtonian fluid on free convection heat and mass transfer from a vertical wall is analyzed by considering double dispersion in a non-Darcy porous medium with constant wall temperature and concentration conditions. The Ostwald–de Waele power law model is used to characterize the non-Newtonian fluid behavior. In this case a similarity solution is possible. The variation of heat and mass transfer coefficients with the governing parameters such as power law index, thermal and solutal dispersion parameters, inertia parameter, buoyancy ratio, and the Lewis number is discussed for a wide range of values of these parameters.

Jet impingement heat transfer on a flat plate at a constant wall temperature

Gas-to-wall heat transfer configuration for a round air jet impinging on a circular flat plate is investigated experimentally to derive an average Nusselt number correlation. The impingement plate is placed at the bottom of a large adiabatic enclosure, and its temperature is imposed, by external circulation of a coolant. The simultaneous measurements of mass flow rate and characteristic temperatures (hot jet, cold wall, enclosure outlet) permit the determination of the average wall heat transfer coefficient, through an enthalpic balance of the enclosure. The jet Reynolds number, nozzle diameter D and nozzle-to-plate distance H are varied. These experimental measurements are compared with the results of a numerical CFD modelling. Simulations under constant wall heat flux conditions are compared to local Nusselt number distributions as given by the current literature, which validates the use of the Shear Stress Transport (SST) k – ω turbulence model for this problem. Simulated Nusselt numbers obtained at a constant wall temperature are found lower than under uniform heat flux conditions. Measurements and simulation results, at a constant wall temperature, are in good agreement. An average Nusselt number correlation is proposed for jet impingement heat transfer calculations under constant wall temperature conditions, as a function of the jet Reynolds number Re j ( 10 000 ⩽ Re j ⩽ 30 000 ) , the geometrical parameters R / D , H / D ( 3 ⩽ R / D ⩽ 10 ; 2 ⩽ H / D ⩽ 6 ), and the dimensionless viscosity ratio μ j / μ w ( 1.1 ⩽ μ j / μ w ⩽ 1.4 ) .

Conjugate Heat Transfer Computation for Evaluation of Single-Blow Method for Compact Fin-Tube Heat Exchangers

Computations have been carried out to evaluate heat transfer coefficients given by the single-blow method that is characterized by a transient and conjugate heat transfer problem between the fluid and the solid. Both heat conduction and convection equations are solved numerically to obtain the transient fluid and fin temperature distributions. Finite volume solutions indicate that the fin temperature distribution of the single-blow method varies with time and position and that the fluid temperature distribution of the single-blow method is close to that observed in the steady-state computation specified with the constant wall temperature condition. It is found that the local heat transfer coefficient resulting from the single-blow method is almost identical to that from the steady-state constant wall temperature computation and is nearly time-independent.

A numerical investigation on effects of ceiling and floor surface temperatures and room dimensions on the Nusselt number for a floor heating system

In this study, the effect of ceiling and floor surface temperatures and room dimensions on the Nusselt number over the floor of a floor heating system has been investigated numerically. The variation of the Nusselt number with Rayleigh number has been analyzed under constant wall temperature condition for different ceiling temperatures (10–25 °C) and room dimensions. It has been seen that when the room dimensions and temperature difference between the ceiling and interior air are increased, the Nusselt number over the floor increases as well. The numerical results have been compared with the correlations given in the literature. It has been seen that the correlations available in the literature are valid only for given thermal conditions and room dimensions. The results calculated from the correlations which do not consider the effects of ceiling and floor surface temperatures deviate up to 35% than the results of this numerical study carried out for different ceiling and floor surface temperatures and room dimensions. Therefore, a new correlation for Nusselt number over the floor, which contain the influence of thermal conditions and all of room dimensions must be discovered.

Heat transfer of gas–solid two-phase mixtures flowing through a packed bed

This paper reports an experimental study on both transient and steady-state heat transfer behavior of a gas–solid two-phase mixture flowing through a packed bed under constant wall temperature conditions. A logarithmic mean temperature difference (LMTD) method is used to process the temperature data to obtain the overall heat transfer coefficient. The influences of particle loading and gas flow Reynolds number are investigated. The results show that the introduction of suspended particles greatly enhances heat transfer between the flowing gas–solid two-phase mixture and the packed bed, and the enhancement increases approximately linearly with increasing particle loading. The heat transfer coefficient data are processed to give the Nusselt number, which is found to correlate well to the Reynolds number, the Archimedes number and the suspended particle loading ratio. A comparison of the data of this work with the published data reveals large discrepancy. Possible reasons for the discrepancy are discussed.

A Study on Boiling Heat Transfer Augmentation of Heating Process Using Ultrasonic Vibrations

The present study was experimentally investigated the effect of ultrasonic vibrations on boiling heat transfer augmentation during the heating process. The experiments were carried out under the constant wall temperature condition and were divided into two cases applying with and without ultrasonic vibrations, respectively. Also, the temperature distributions in a vessel filled with water were measured using thermocouples during the heating process, heat transfer coefficient and augmentation ratio of heat transfer on states of convection, subcooled boiling and saturated boiling were calculated from obtained temperature profiles. In addition, the profiles of pressure variations measured using a hydrophone were compared with the augmentation ratio of heat transfer in acoustic fields. The results of experimental study were revealed that general profiles of heat transfer coefficient and augmentation ratio of heat transfer is more increased the convection state than the others states. Moreover, the profiles of acoustic pressure is relatively higher near ultrasonic transducer than other points where is no installed it and affects the augmentation ratio of heat transfer. In the end, as well as known “acoustic streaming” induced by ultrasonic vibrations is one of the prime effects acoustically augmented boiling heat transfer or phase change heat transfer.

A study of steam methanol reforming in a microreactor

Steam reforming of methanol is investigated numerically considering both heat and mass transfer of the species in a packed bed microreactor. The numerical results are shown to be in good agreement with experimental data [M.T. Lee, R. Greif, C.P. Grigoropoulos, H.G. Park, F.K. Hsu, J. Power Sources Transport in, 166 (2007) 194–201] with a BASF F3-01(CuO/ZnO/Al 2O 3) catalyst. A correlation for the conversion efficiency of methanol has been obtained as a function of the operating temperature and a dimensionless time parameter which represents the ratio of the characteristic time of the methanol flow to the time for chemical reaction. The results show that for the constant wall temperature condition the steam reforming process of methanol results in a nearly uniform temperature throughout the microreactor over the range of operating conditions.

Use of Streamwise Periodic Boundary Conditions for Problems in Heat and Mass Transfer

Fully developed periodic boundary conditions have frequently been employed to effect performance calculations for heat and mass exchange devices. In this paper a method is proposed, which is based on the use of primitive variables combined with the prescription of slip values. Either pressure difference or mass flow rate may be equivalently prescribed. Both constant wall temperature (Dirichlet) and constant heat flux (Neumann) conditions may be considered, as well as the intermediate linear (Robin) boundary condition. The example of an offset-fin plate-fin heat exchanger is used to illustrate the application of the procedure. The mathematical basis by which the method may be extended to the consideration of mass transfer problems with arbitrary boundary conditions, and associated continuity, momentum, and species sources and sinks is discussed.

Heat transfer of gas–solid two-phase mixtures flowing through a packed bed under constant wall heat flux conditions

Abstract An experimental study has been carried out on both the transient and steady state heat transfer of a gas–solid two-phase mixture flowing through a cylindrical packed bed reactor under the constant wall heat flux conditions. A logarithmic mean temperature difference (LMTD) method is used to process the steady state data to obtain the overall heat transfer coefficient. The effects of solids loading, particle size and flow Reynolds number are investigated. The results show that the introduction of suspended particles greatly enhances the heat transfer and the enhancement increases approximately linearly with solids loading and the effect of particle size is relatively weak under the conditions of this work. A correlation is proposed based on the experimental data, which relates well the Nusselt number to the Reynolds number, the Archimedes number and the suspended solids loading. Given other conditions, the Nusselt number at the constant wall heat flux conditions is much higher than that under the constant wall temperature conditions. It is shown that the Reynolds number and particle loading have a greater influence on the Nusselt number under the constant heat flux conditions than that under the constant wall temperature conditions investigated by the authors in a previous study.

CO 2 and R410A flow boiling heat transfer, pressure drop, and flow pattern at low temperatures in a horizontal smooth tube

Flow boiling heat transfer coefficient, pressure drop, and flow pattern are investigated in the horizontal smooth tube of 6.1 mm inner diameter for CO 2, R410A, and R22. Flow boiling heat transfer coefficients are measured at the constant wall temperature conditions, while pressure drop measurement and flow visualization are carried out at adiabatic conditions. This research is performed at evaporation temperatures of −15 and −30 °C, mass flux from 100 to 400 kg m −2 s −1, and heat flux from 5 to 15 kW m −2 for vapor qualities ranging from 0.1 to 0.8. The measured R410A heat transfer coefficients are compared to other published data. The comparison of heat transfer coefficients for CO 2, R410A, and R22 is presented at various heat fluxes, mass fluxes, and evaporation temperatures. The difference of coefficients for each refrigerant is explained with the Gungor and Winterton [K.E. Gungor, R.H.S. Winterton, A general correlation for flow boiling in tubes and annuli, Int. J. Heat Mass Transfer 29 (1986) 351–358] correlation based on the thermophysical properties of refrigerants. The Wattelet et al. [J.P. Wattelet, J.C. Chato, B.R. Christoffersen, J.A. Gaibel, M. Ponchner, P.J. Kenny, R.L. Shimon, T.C. Villaneuva, N.L. Rhines, K.A. Sweeney, D.G. Allen, T.T. Heshberger, Heat Transfer Flow Regimes of Refrigerants in a Horizontal-tube Evaporator, ACRC TR-55, University of Illinois at Urbana-Champaign, 1994], and Gungor and Winterton [K.E. Gungor, R.H.S. Winterton, A general correlation for flow boiling in tubes and annuli, Int. J. Heat Mass Transfer 29 (1986) 351–358] correlations give the best agreement with the measured heat transfer coefficients for CO 2 and R410A. Pressure drop for CO 2, R410A, and R22 at various mass fluxes, evaporation temperatures and qualities is presented in this paper. The Müller-Steinhagen and Heck [H. Müller-Steinhagen, K. Heck, A simple friction pressure drop correlation for two-phase flow in pipes, Chem. Eng. Process. 20 (1986) 297–308], and Friedel [L. Friedel, Improved friction pressure correlations for horizontal and vertical two-phase pipe flow, in: The European Two-Phase Flow Group Meeting, Ispra, Italy, 1979 (paper E2)] correlation can predict most of the measured pressure drop within the range of ±30%. The relation between pressure drop and properties for each refrigerant is described by applying the Müller-Steinhagen and Heck correlation. The observed two-phase flow patterns for CO 2 and R410A are presented and compared with flow pattern maps. Most of the flow patterns can be determined by the Weisman et al. [J. Weisman, D. Duncan, J. Gibson, T. Crawford, Effect of fluid properties and pipe diameter on two-phase flow patterns in horizontal lines, Int. J. Multiphase Flow 5 (1979) 437–462] flow pattern map.

A thermal periodic boundary condition for heating and cooling processes

A thermal periodic boundary condition is developed to compute heat transfer in spatially periodic domains. The condition is formulated on the basis of thermal similarity and is implemented within the framework of a mass flow-based periodic condition for the flow field. The thermal periodic boundary condition is validated by computing the flow and heat transfer for fully developed pipe flow under constant heat flux and constant wall temperature conditions, and for a generic spatially periodic geometry.

Heat transfer of gas flow through a packed bed

This paper reports an experimental study of both the transient and steady-state heat transfer behaviour of a gas flowing through a packed bed under the constant wall temperature conditions. Effective thermal conductivities and convective heat transfer coefficient are derived based on the steady-state measurements and the two-dimensional axial dispersion plug flow (2DADPF) model. The results reveal a large temperature drop at the wall region and the temperature drop depends on the axial distance from the inlet. The 2DADPF model predicts the axial temperature distribution fairly well, but the prediction is poor for the radial temperature distribution. Length-dependent behaviour of the effective heat transfer parameters and non-uniform flow behaviour are proposed to be responsible. A comparison with previously published correlations and data in the literature shows that the relationships proposed by Bunnell et al. and Demirel et al. agree well with the measured effective radial thermal conductivity, whereas the wall–fluid heat transfer coefficient is better represented by the Li–Finlayson correlation.

풀 핵비등시 단일 기포 성장에 대한 벽면 과열도의 영향에 관한 연구

Nucleate pool boiling experiments for R11 under a constant wall temperature condition were carried out. A microscale heater array was used for the heating and the measurement of high temporal and spatial resolution by the Wheatstone bridge circuit. Very sensitive heat flow rate data were obtained by the control for the surface condition with high time resolution. The measured heat flow rate shows a discernable peak at the initial growth stage and reaches an almost constant value. In the thermal growth region, bubble shows a growth proportional to . The bubble growth behavior is analyzed with a dimensionless parameter to compare with the previous results in the same scale. As the wall superheat increases, the departure diameter and the departure time increase, and the waiting time decreases. But the asymptotic growth rate is not affected by the wall superheat change. The effect of the wall superheat is resolved into the suggested growth equation. Dimensionless parameters of time and bubble radius characterize the thermal growth behavior well, irrespective of wall condition. The comparison between the result of this study and the previous results shows a good agreement at the thermal growth region. The quantitative analysis for the heat transfer mechanism is conducted with the measured heat flow rate behavior and the bubble growth behavior. The required heat flow rate for the volume change of the observed bubble is about twice as much as the instantaneous heat flow rate supplied from the wall.