Transient Forced Convection Heat Transfer Research Articles

Influence of inclined magnetic field and cross diffusion on transient forced convective heat and mass transfer flow along a porous wedge with variable Prandtl number

The unsteady 2-D forced convective mass and heat transfer flow of an incompressible, viscous and electrically conducting fluid along a porous wedge is examined in this article using numerical analysis to survey the effects of a magnetic field and thermophoresis. This is done in the existence of temperature-dependent thermal conductivity, a variable Prandtl number, and the effect of cross diffusion. The governing nonlinear PDEs have been converted into nonlinear ODEs by using a similarity transformation. The modified ODEs are solved for similar solutions using the shooting technique and the Runge–Kutta fourth-order method. The temperature, velocity, concentration, thermophoretic velocity, and thermophoretic particle deposition velocity results for various parameters are provided. The Dufour number’s effect enhances the temperature profile and rate of heat transfer. Moreover, it is noted that the rate of mass transfer is meaningfully influenced by the variation of the thermophoresis parameter and Soret number. Thus, in any physical model where the thermal conductivity of the fluid is temperature dependent, the Prandtl number within the BL have to be treated as variable rather than constant.

Analysis of transient laminar forced convective heat transfer in parallel-plate channels with time-varying thermal boundary conditions of the inlet and wall

The problem of transient laminar forced convective heat transfer in parallel-plate channels with time-varying thermal boundary conditions of the inlet and wall is thoroughly investigated in this study. By employing the methodologies of generalized integral transform technique (GITT) and Duhamel's theorem, we successfully derive an analytical solution for this problem. To validate the accuracy of our analytical findings, a comparison is made with the corresponding numerical solution, demonstrating a remarkable level of agreement between the two approaches. In order to obtain the general patterns of heat transfer, we employ a decoupling technique to separate the thermal response in the case of harmonic temperature boundary conditions of the inlet and wall. This approach enables us to uncover the intricate relationship between the weight coefficient of heat input and the two-dimensional spatial location of the channel, as well as the dimensionless angular frequency of heat input. Furthermore, we present the corresponding dimensionless cut-off angular frequency of heat input at various locations within the channel. To gain further insights, we conduct an in-depth analysis of the effects of integration time or distance on the dimensionless optimum angular frequency corresponding to the dimensionless maximum average heat flux.

Heat transfer past rotationally oscillating circular cylinder heated with time‐periodic pulsating temperature in a uniform flow

AbstractThe transient forced convection heat transfer from an impulsively started rotationally oscillating circular cylinder, heated with time‐periodic pulsating temperature in a two‐dimensional uniform, laminar and viscous flow of Newtonian, incompressible, constant property fluid is investigated in this paper. The two‐dimensional Navier‐Stokes and energy equations are solved using a higher‐order compact (HOC) finite difference scheme on non‐uniform polar grids. Computational code for solving the problem is written in C programming language and is validated against existing research works. The flow and heat transfer characteristics are mainly influenced by the amplitude of time‐periodic pulsating temperature (), period of time‐periodic pulsating temperature (), Prandtl number (), Reynolds number (), and cylinder oscillation parameters i.e. maximum rotational rate (), frequency ratio (). The numerical computations are done for the range of parameter values , , , , and . The backward heat transfer is observed for certain values of the amplitude of time‐periodic pulsating temperature and the amount of backward heat transfer enhances with increasing amplitude of temperature. High period value () of pulsating temperature is found to reduce the amount of heat convection downstream. The influence of pulsating temperature is discussed for an increase or decrease in heat transfer rate relative to the constant cylinder‐temperature case.

Transient forced convective heat transfer of helium gas in a narrow tube heated by exponential time-varying heat source

ABSTRACT The analysis and evaluation of transient heat transfer are essential to predict stability and safety of helium gas cooling fusion blanket due to plasma disruption. In this study, transient turbulent heat transfer characteristics of helium gas flowing in a narrow tube were experimentally investigated by using a forced convection gas loop. A narrow tube with an inner diameter of 1.8 mm was employed as test heater and heated exponentially increasing with time under various e-folding time of heat generation rate (0.04–15 s), flow velocities (101–248 m/s), inlet gas temperatures (293–313 K), and pressures (298–502 kPa). The experimental results showed that the heat transfer process can be separated into transient-state heat transfer and quasi-steady-state one at the e-folding time of near 1.5 s. The dependence of quasi-steady-state heat transfer on gas pressure and flow velocity was significant, but the transient-state one was less dependent on gas pressure and flow velocity. The influence of inlet gas temperature on the heat transfer coefficient was not obvious under both transient and quasi-steady conditions. A semi-empirical correlation of the relationship between transient-state and steady-state heat transfer was developed based on substantial experimental data by employing a nondimensional parameter of Fourier number.

Transient heat transfer for helium gas flowing over a horizontal cylinder in a narrow channel

ABSTRACTTransient forced convection heat transfer due to exponentially increasing heat input to a heater is important as a database for the safety assessment of the transient heat transfer process in a very high temperature gas-cooled reactor (VHTR). Forced convection transient heat transfer for helium gas flowing over a cylinder in a narrow channel was experimentally studied at various periods of exponential increases in heat input. The test heater was mounted horizontally along the center part of a circular test channel with a diameter of 5 mm. The heat generation rates of the heater,, was increased with the exponential function, . By using a narrow channel, relatively high flow velocity was achieved, and experimental data at a high Reynolds number were obtained. According to the results, the surface temperature and the heat flux exponentially increased with time. It was clarified that the heat transfer coefficient approached the quasi-steady-state for a period of more than approximately 1 s, and it reached higher values for a period of less than approximately 1 s. The heat transfer coefficients present high dependence on the flow velocity of the helium gas and the heater diameter. Heat transfer correlations at the quasi-steady state and the transient state were obtained based on the experimental data.

Steady and transient forced convection heat transfer for water flowing in small tubes with exponentially increasing heat inputs

Steady and transient heat transfer coefficients for water flowing in small tubes with exponentially increasing heat inputs were measured. Platinum tubes with inner diameters of 1.0 and 2.0 mm were used as test tubes, which were mounted vertically in the experimental water loop. In the experiment, the upward flow velocity ranged from 2 to 16 m/s, and the corresponding Reynolds numbers ranged from 4.77 × 103 to 9.16 × 104 at the inlet liquid temperatures ranged from 298 to 343 K. The heat generation rate exponentially increased with the function. The period of the heat generation rate ranged from 24 ms to 17.5 s. Experimental results indicate that steady heat transfer coefficients decreased with the increase in the inner diameter of the small tube. Moreover, the ratio of bulk viscosity to near-wall viscosity of water increased with the rise in surface temperature of the vertical tube. From the experimental data, correlations of steady-state heat transfer for inner diameters of 1.0 and 2.0 mm were obtained. The heat transfer coefficient increased with decreasing the period of the heat generation rate as the flow velocity decreased. Moreover, the Nusselt number under the transient condition was affected by the Fourier number and the Reynolds number.

Artificial Neural Network Approach for Transient Forced Convective Heat Transfer Optimization

This present research uses artifical neural networks (ANNs) to analyze and estimate the influence of transfer functions and training algorithms on experimentally determined Nusselt numbers, friction factors, entropy generation numbers and irreversibility distribution ratios for nine different baffle plate inserted tubes. Nine baffle-inserted tubes have several baffles with various geometric parameters used in the experiments with a baffle area blockage ratio of two, with different pitch to diameter ratios, different baffle orientation angles and different baffle spacings. The actual experimental data sets were used from previous author’s studies and applied as a input data set of ANNs. MATLAB toolbox was used to search better network configuration prediction by using commonly used multilayer feed-forward neural networks (MLFNN) with back propagation (BP) learning algorithm with thirteen different training functions with adaptation learning function of mean square error and TANSIG transfer function. In this research, eighteen data samples were used in a series of runs for each nine samples of baffle-inserted tube. Reynold number, tube lenght to baffle spacing ratio, baffle orientation angle and pitch to diameter ratio were considered as input variables of ANNs and the time averaged values of Nusselt number, friction factor, entropy generation number and irreversibility distribution ratio were determined as the target data. The total 70% of the experimental data was used to train, 15% was used to test and the rest of data was used to check the validity of the ANNs. The TRAINBR training function was found as the best model for predicting the target experimental outputs. Almost perfect accuracy between the neural network predictions and experimental data was achieved with mean relative error (MRE) of 0,000105816% and correlation coefficient (R) that was 0,999160176 for all datasets, which suggests the reliability of the ANNs as a strong tool for predicting the performance of transient forced convective heat transfer applications.

Transient forced convection heat transfer for nitrogen gas flowing over plate heater with exponentially increasing heat input

The transient forced convection heat transfer of plate heater for nitrogen gas due to exponentially increasing heat input was investigated experimentally and numerically. The platinum ribbon with a thickness of 0.1mm and a width of 4.0mm was used as the test heater. The heat generation rate of the test heater was raised with exponential function. The inlet flow velocity was ranged from 2 to 4m/s for the gas temperature of 313K under the system pressure of 500kPa. The period of heat generation rate was ranged from 45ms to 8s. Experimental results indicate that the surface temperature difference and heat flux increased exponentially as the heat generation rate increased through an exponential function. It was clarified that the heat transfer coefficient was divided into the two regions for the period ranging from 45ms to 8s. The numerical results were compared with experimental data. The numerical simulation was in agreement with the experimental data.

Experimental study on transient heat transfer enhancement from a twisted plate in convection flow of helium gas

The knowledge of heat transfer enhancement using a heater with twisted configuration is important for the high performance design of high temperature thermal systems such as very high temperature reactors (VHTR). In this study, forced convection transient heat transfer for helium gas at various periods of exponential increase of heat input to a twisted plate was experimentally studied. The heat generation rate, Q̇ , was raised with exponential function, Q̇=Q0exp(t/τ) (where t is time, and τ is period of heat generation rate). The transient heat transfer experimental data were measured for the periods ranged from 46ms to 17s and at gas temperatures of 303K and 313K under 500kPa. The flow velocities ranged from 4m/s to 10m/s. The pitches of the twisted plates were 20mm, 25mm, and 30mm. The surface temperature and heat flux increased exponentially with the time. It was clarified that the heat transfer coefficient approaches the quasi-steady-state one for the period longer than about 1s, and it becomes higher for the period shorter than about 1s. An enhancement in the heat transfer performance for the twisted plate was clarified. Based on the experimental data, empirical correlation of heat transfer for twisted plate was obtained. Furthermore, the mechanism of heat transfer enhancement due to twisted plate was stated in detail.

Width effect and local transient heat transfer characteristics for helium gas flowing over a twisted plate

Forced convection transient heat transfer for helium gas at various periods of exponential increase of heat input to a short twisted plate with various helix angles was experimentally studied. Twisted plates were made of platinum plate with a thickness of 0.1 mm and width of 2 and 4 mm. The heat generation rates of the heater was raised with exponential function, , where t is time, and τ is period of heat generation rate. The transient heat transfer experimental data were measured for the periods ranged from 80 ms to 17 s and at a gas temperature of 303 K under 500 kPa. The flow velocities ranged from 4 to 10 m/s. In the experiments, the twisted plates with different width were tested. The local mean heat transfer coefficients have been tested as well. An enhancement in the heat transfer coefficient for the twisted plate with different width was clarified. And also, the heat transfer coefficients for the first half pitch are 24% higher than that for the total length of the same twisted plate.

Unsteady Laminar Forced-Convective Tube Flow Under Dynamic Time-Dependent Heat Flux

A new all-time model is developed to predict transient laminar forced convection heat transfer inside a circular tube under arbitrary time-dependent heat flux. Slug flow (SF) condition is assumed for the velocity profile inside the tube. The solution to the time-dependent energy equation for a step heat flux boundary condition is generalized for arbitrary time variations in surface heat flux using a Duhamel's integral technique. A cyclic time-dependent heat flux is considered and new compact closed-form relationships are proposed to predict (i) fluid temperature distribution inside the tube, (ii) fluid bulk temperature and (iii) the Nusselt number. A new definition, cyclic fully developed Nusselt number, is introduced and it is shown that in the thermally fully developed region the Nusselt number is not a function of axial location, but it varies with time and the angular frequency of the imposed heat flux. Optimum conditions are found which maximize the heat transfer rate of the unsteady laminar forced-convective tube flow. We also performed an independent numerical simulation using ansys fluent to validate the present analytical model. The comparison between the numerical and the present analytical model shows great agreement; a maximum relative difference less than 5.3%.

1308 発熱率が異なる平板発熱体のヘリウムガス強制対流過渡熱伝達

1308 発熱率が異なる平板発熱体のヘリウムガス強制対流過渡熱伝達

1208 種々のガスを用いた強制対流過渡熱伝達に関する研究(GS-6 熱工学(2))

1208 種々のガスを用いた強制対流過渡熱伝達に関する研究(GS-6 熱工学(2))

Transient Turbulent Flow and Heat Transfer Phenomena in Plate-Fin Type Cross-Flow Heat Exchanger

In this article, a transient performance of a plate-fin cross-flow heat exchanger with convergent–divergent longitudinal vortex generators is investigated. The effect of flow geometry is taken into account to analyze the transient forced convection heat transfer in a designed heat exchanger. The time-dependent Nusselt number and dissipation energy criterion are experimentally measured in 4- and 8-kW heater powers for various Reynolds numbers between 42,000 and 60,000 for the hot and cold fluids. In order to present the quality of the heat exchanger, the general empirical equations of the time-dependent Nusselt number and friction factor were derived as a function of the Reynolds number corresponding to fin geometry parameters. Following this, the transient behavior of the heat exchanger according to the change in the inlet and outlet temperatures of the hot and cold fluids was analyzed. The results showed that the variations of the time-dependent dissipation energy criterion increase with the increase in the Reynolds number. The appropriate correlations are proposed to predict the heat transfer and friction characteristics of the transient performance for the presented configuration, which indicates the designed heat exchanger has good heat conduction.

Effect of Heater Configurations on Transient Heat Transfer for Various Gases Flowing over a Twisted Heater

Forced convection transient heat transfer coefficients were measured for helium gas and carbon dioxide gas flowing over a twisted heater due to exponentially increasing heat input. The twisted platinum ribbon with a thickness of 0.1 mm was used as test heater and heated by electric current. The heat generation rate was exponentially increased with a function of Q0exp(t/τ). The gas flow velocities ranged from 1 to 10 m/s, the gas temperatures ranged from 313 to 353 K, and the periods of heat generation rate ranged from 46 ms to 17 s. The surface temperature difference and heat flux increase exponentially as the heat generation rate increases exponentially. Transient heat transfer coefficients increase with increasing gas flow velocity. The heater configuration of twisted heater in this study shows a large effect on the heat transfer coefficient. Empirical correlations for quasi-steady-state heat transfer were obtained based on the experimental data.

Thermal analysis of transient laminar flow past an in-line cylinders array containing phase change material

Transient laminar flow past an in-line cylinders array containing phase change material (PCM) is simulated using the finite-element method. The problem of the PCM in the cylinders and transient forced convection heat transfer simulates the thermal behaviour of an energy storage system. A parametric study of heat exchanges between the PCM and flow (expressed by the Nusselt number) is reported for Reynolds numbers between 125 and 1000. The pitch to diameter ratios (PDRs) are varied and the Prandtl number is fixed at 0.71. The PCM melting time is predicted for the examined range of Reynolds numbers and PDRs. Plots of the velocity flow field and isotherms are presented. Results are compared favourably with published numerical results. The results indicated that the Reynolds number has a significant effect on the PCM melting time, whereas the PDR has an insignificant effect.

Transient Forced Convection Heat Transfer due to Exponentially Increasing Heat Input for Helium Gas Flowing on a Narrow Plate

Steady and transient forced convection transient heat transfer due to exponentially increasing heat input to a heater is important as a database for safety assessment of the transient heat transfer process not only in a high temperature gas cooled reactor (HTGR) due to an accident in excess reactivity but also in high heat flux gas cooling devices such as a gas turbine and a rocket engine. In this research, forced convection transient heat transfer for helium gas at various periods of exponential increase of heat input (Q0 exp(t/τ)) to a horizontal narrow plate was numerically solved based on a turbulent flow model. The platinum plate with a length of 50 mm was used as test heater. The velocities ranged from 4 to 10 m/s, the gas temperatures ranged from 313 to 353 K, and the periods of heat generation rate, τ, ranged from 46 ms to 8.6 s. The values of numerical solutions for surface temperature and heat flux were compared and discussed with authors’ experimental values. It was obtained that the surface temperature difference and heat flux increase exponentially as the heat generation rate increases with the exponential function. Then the temperature within the boundary layer also increases with the increase of the surface temperature. It is understood that the gradient of the temperature distribution near the wall of the plate is higher at a higher surface temperature difference. The values of numerical solutions for surface temperature and heat flux at the velocity of 6 m/s agree well with the experimental data, though they show some differences at other velocities. And also, heat transfer coefficients at the velocity of 6 m/s agree well with the experimental data, though they show some differences at other velocities. They agree within 15% at various periods and velocities.Copyright © 2008 by ASME

1212 増加熱入力によるねじり発熱体の強制対流過渡熱伝達に関する実験的研究(GS-6 伝熱促進)

1212 増加熱入力によるねじり発熱体の強制対流過渡熱伝達に関する実験的研究(GS-6 伝熱促進)

Theoretical and Experimental Studies on Transient Forced Convection Heat Transfer of Helium Gas

Theoretical and Experimental Studies on Transient Forced Convection Heat Transfer of Helium Gas

Transient Laminar Forced Convection Heat Transfer of power-law Fluids along a Wavy Plate

The transient laminar forced convection of power-law fluids along a wavy surface is studied by the coordinate transformation and the cubic spline collocation method. Effects of the power-law index and the amplitude-wavelength ratio on the Nusselt number and the skin-friction coefficient are examined. The fluid with larger power-law index has larger transient local Nusselt numbers at the leading edge; as the fluid moves downstream from the leading edge, the transient local Nusselt number and the transient mean Nusselt number of dilatant fluids decrease rapidly while those of pseudoplastic fluids increase. The transient local skin-friction coefficient decreases as the power-law index is increased. Moreover, the harmonic curves for the transient local Nusselt number and the transient local skin-friction coefficient have the same wave number as that of the wavy surface.