Instantaneous Heat Transfer Rates Research Articles

Experimental comparative study on thermal performance of latent heat storage tanks with pin, perforated, and rectangular fins at different orientations

The low thermal conductivity of phase change materials (PCMs) has limited their widespread use in practical applications. In the present study, different fin structures, namely, rectangular, perforated, and pin were examined to analyze the thermal performance of the melting process in rectangular latent heat storage tanks. Experiments were performed at both horizontal and vertical orientations to evaluate the effectiveness of different fin configurations. Visual observation of the phase change evolution at different time intervals was enabled through a transparent plexiglass shell. Instantaneous heat transfer rate and energy storage were measured using thermocouple readings and melting photographs. The results show that the maximum heat transfer coefficient between the heated wall and PCM is obtained by the pin-finned tank followed by perforated and rectangular-finned tanks. This thermal behavior is justified by the intensification of the upward convection flows through the voids provided by pin fins or perforated fins. Although the rectangular fin structure has the lowest convective heat transfer coefficient, its heat transfer rate is slightly higher than the other structures due to its larger heat transfer area. At a wall temperature of 70℃, the convective heat transfer coefficient and heat transfer rate obtained by the pin fin configuration are respectively 25% higher and 4% less than those of the rectangular fin. It reveals that the pin fin structure provides the most effective heat transfer area compared to its counterparts which have a significantly larger fin volume. In addition, it was found that regardless of the fin configuration, the melting rate in the horizontal tank was significantly higher than in the vertical tank due to the formation of more vortical flow structures within the molten PCM. The melting time in the unfinned horizontal tank was less than those of the vertical finned tanks implying that the tank orientation should be well-chosen to minimize the melting time along with adding fins of various configurations.

Numerical investigation of transient thermal behaviour of fully wet and porous moving semi-spherical fin

A variety of engineering applications demand efficient and adaptable fin structures for the intensification of heat exchange. The semi-spherical fin structures are useful in the field of refrigeration, chemical processing systems, aerospace etc. In this regard, the present article numerically investigates the transient thermal behaviour of a fully wet semi-spherical fin. The study incorporates the Darcy model as the fin is made up of porous material. Further, the fin is exposed to convective-radiative heat exchange and is subject to uniform motion. The heat balance equation has been reduced to get a nonlinear partial differential equation (PDE) which is computed by employing the finite difference method (FDM). The dimensionless terms are grouped together and their influence on the temperature distribution in a semi-spherical fin is studied. Also, the instantaneous heat transfer rate and the transient fin efficiency have been modelled and their variations with relevant parameters have been graphically depicted. And these are found be strong functions of Peclet number, wet porous nature and dimensionless time. As a main outcome the semi-spherical fin efficiency is positively influenced by the Peclet number. Along with the fundamental point of interest the results presented benefit the fin designing purposes.

Large eddy simulation of enhanced heat transfer in pulsatile turbulent channel flow

Heat transfer in pulsatile turbulent channel flow is investigated by means of Large Eddy Simulation. Incompressible flow within a periodic computational domain is driven by a pulsating axial pressure gradient at a turbulent Reynolds number of Reτ=350. A localized dynamic sub-grid scale approach is chosen to model unclosed stress terms. A layer-averaged sub-grid model determines turbulent Prandtl numbers that depend on wall distance. Compared to the existing literature, a much wider range of oscillation parameters is studied. In particular, forcing frequencies correspond to Womersley numbers from Wo=14 to 70, while forcing amplitudes reach values that result in strongly pronounced flow reversal, i.e. reverse flow velocities up to five times larger than the mean flow velocity.At moderate pulsation amplitudes, i.e. in the range of emerging flow reversal, strong deviations of instantaneous heat transfer rates from the temporal mean are observed. Particularly at times of flow reversal, an increase in heat transfer up to 60% above non-pulsatile values is observed. However, when averaging over a complete cycle, any enhancement in mean heat transfer is only minor.On the other hand, simulations at larger pulsations amplitudes that result in strongly pronounced flow reversal, show a enhancement of mean heat transfer in excess of 100%. Note that such significant enhancement of heat transfer has not been reported previously in Large Eddy Simulation of turbulent pulsatile flow. The paper offers a physical interpretation of the results and concludes that an overall increase in turbulent transport is responsible for the observed significant enhancement of convective heat transfer.

Heat Transfer Enhancement by Sinusoidal Motion of a Water-Based Nanofluid

The oscillatory flows are often utilized in order to augment heat transfer rates in various industrial processes. It is also a well-known fact that nanofluids provide significant enhancement in heat transfer at certain conditions. In this research, heat transfer in an oscillatory pipe flow of both water and water–alumina nanofluid was studied experimentally under low frequency regime laminar flow conditions. The experimental apparatus consists of a capillary tube bundle connecting two reservoirs, which are placed at the top and the bottom ends of the capillary tube bundle. The upper reservoir is filled with the hot fluid while the lower reservoir and the capillary tube bundle are filled with the cold fluid. The oscillatory flow in the tube bundle is driven by the periodic vibrations of a surface mounted on the bottom end of the cold reservoir. The effects of the frequency and the maximum displacement amplitude of the vibrations on thermal convection were quantified based on the measured temperature and acceleration data. It is found that the instantaneous heat transfer rate between de-ionized (DI) water (or the nanofluid)-filled reservoirs is proportional to the exciter displacement. Significantly reduced maximum heat transfer rates and effective thermal diffusivities are obtained for larger capillary tubes. The nanofluid utilized oscillation control heat transport tubes achieve high heat transfer rates. However, heat transfer effectiveness of such systems is relatively lower compared to DI water filled tubes.

Local instantaneous heat transfer around a single elongated bubble in inclined pipes

The instantaneous convective heat transfer rate resulting from the passage of a single elongated bubble propagating in an inclined pipe with a constant translational velocity is examined experimentally for various pipe inclinations and liquid flow rates. Local instantaneous heat transfer rates in the upper and lower parts of the pipe were determined using wall temperature measurements by an IR camera. The ensemble-averaged values of the heat transfer rate are presented as a function of the distance from either the bubble tip or bottom. Optical sensors synchronized with the IR camera were used for this purpose. Possible mechanisms that govern the enhancement of the heat transfer rate during the passage of an elongated bubble are discussed.

Experimental and numerical investigation of solar flat plate cooking unit for domestic applications

The continuous technological development in the field of solar thermal application is a pressing need to harvest and utilize the solar energy in an efficient manner. Development of a flat plate cooking unit (FPCU) intended for the indirect mode of solar cooking has been attempted. In this paper, the performance of the FPCU integrated with the latent heat thermal energy storage system is studied experimentally in order to replace the conventional mode of biomass or LPG based cooking. The heat transfer fluid (HTF), Therminol 55 and the phase change material (PCM), D Mannitol is used in the experimental set-up. The food medium energy utilization, instantaneous heat transfer rate and detailed energy balance for the cooking unit integrated with storage tank is reported. In addition, computational fluid dynamic (CFD) analysis using ANSYS Fluent software is also performed on the newly developed FPCU to analyze the cooking period heat transfer behavior. The results of CFD analysis is validated with experimental results and the results show that the average heat transfer coefficient during the cooking process is found to be approximately 100 W/m2K.

An Analytical Technique of Transient Phase-Change Material Melting Calculation for Cylindrical and Tubular Containers

ABSTRACTIn this study, an analytical model for a class of heat storage that utilizes latent heat of a phase-change material (PCM) is developed. Two basic shell-and-tube configurations are considered, one in which the PCM melts inside the tubes while the heat transfer fluid (HTF) flows in the shell along it, and the other in which HTF flows inside the tubes while PCM melts outside. A system of partial differential equations, which describes heat transfer and melting of the PCM and heat transfer in the HTF, is derived with some simplifying assumptions, while still capturing and preserving the essential features of the processes involved. These equations are solved analytically, yielding the overall heat exchange parameters, like instantaneous heat transfer rate, stored energy, and overall operation time of the system. The present work shows that the use of the proposed analytical technique and its modifications for the practical PCM arrangements is beneficial. Proper application of the model makes it possible to obtain the parameters of a real PCM melting process in the form of algebraic formulas, both for the transient values of variables over time, and for the overall process characteristics. A comparison with the results of numerical calculations of transient melting, made using computational fluid dynamics, confirms the validity of analytical findings and allows to assess the degree of accuracy of the results of our analytical method in various practical cases.

Effect of direct evaporative cooling during the charging process of phase change material based storage system for building free cooling application—A real time experimental investigation

The adoption of phase change material (PCM) based free cooling technology in all the climatic conditions is challenging because of its site-specific nature. In order to support the implementation of free cooling concept in unfavorable climatic condition, a novel hybrid system with the combination of evaporative cooling and PCM based free cooling is proposed herewith. The present experimental work aims to compare and analyze the charging behavior of the PCM-based free cooling system without and with the integration of direct evaporative cooling (DEC) unit during the summer month (April) in an Indian city possessing hot–dry climate. By considering the local ambient conditions, PCM with a phase change temperature of 27–29°C is opted for the present study. The time-temperature history of PCM, instantaneous heat transfer rate, charging efficiency, heat exchange between the storage tank and ambient were analyzed and the respective results are presented. It is inferred from the experiments conducted without the integration of DEC unit that PCMs did not undergo phase transition and remained in the liquid phase even after 8h of the experimentation due to unfavorable local ambient conditions. However, in the case of experiments conducted with the integration of DEC unit, PCMs at all locations in the thermal energy storage (TES) tank got solidified irrespective of local ambient temperature and reached its freezing temperature. This is due to the reduction in heat transfer fluid (HTF) temperature and the additional temperature driving potential achieved through DEC. Better results in terms of charging duration was obtained for the experiments conducted with higher temperature driving potential than the higher HTF velocities. The main inference from the present study is that it is much possible to store the useful energy in PCM even during the summer season in the hot and dry climatic zone irrespective of local ambient temperature through the integration of DEC unit with the PCM based free cooling system.

Application of a two phase lattice Boltzmann model in simulation of free surface jet impingement heat transfer

In this paper a two-phase lattice Boltzmann model, capable of handling large density jumps, is used to study the free surface jet impingement cooling in the non-boiling regime. The multiple-relaxation-time (MRT) collision operator is employed to enhance the numerical stability of the model at high Reynolds numbers. The capability of the outlined two-phase LB scheme in accurate capturing of the interface shape is assessed through relevant test cases. These include the oscillating drop, Rayleigh–Taylor instability, and Kelvin–Helmholtz instability. In addition to demonstrate the validity of the model in simulation of heat transfer the predicted distribution of the Nu number on the impingement plate are compared with those of analytical results. The validated numerical solver then is employed to study a planar liquid jet emanating from a nozzle into a quiescent gas and impinging the opposite plate. The influence of the jet Re number and liquid-to-gas density ratio on the instantaneous flow field, interface shape, and heat transfer rate is also examined. This work reports for the first time the simulation of an impinging free surface liquid jet using a two phase lattice Boltzmann model.

Instantaneous heat transfer rate around consecutive Taylor bubbles

While many studies examine the hydrodynamics of two-phase gas–liquid slug flow, the details of the heat transfer mechanism of this flow regime remain largely unknown. Analyzing the detailed hydrodynamics around a single Taylor bubble provides a basis for understanding the heat transfer mechanism. Propagation of a single Taylor bubble is approximately steady; however, in undeveloped slug flow, acceleration and coalescence of consecutive bubbles is unsteady and highly complicated. The present study is aimed to investigate the effects of developing slug flow, particularly in the region where consecutive bubbles coalesce, on the instantaneous heat transfer for different flow regimes and heating locations. To enable controlled flow conditions, experiments were carried out for two consecutive Taylor bubbles rising in a vertical pipe. Measurements of the instantaneous heat transfer coefficient as a function of the leading bubble location were carried out for different separation distances between the two consecutive bubbles. These measurements were performed for various liquid flow rates, corresponding to laminar, transitional and turbulent background flow regimes. The experiments were conducted at several locations corresponding to different heating lengths. Furthermore, a comparison between the heat transfer coefficients of a bubble that has undergone coalescence with those corresponding to a single Taylor bubble with a similar length was performed. For all conditions, it was found that the heat transfer rate is augmented in the presence of an accelerated trailing bubble. The variation of the heat transfer rates is discussed with relation to the local flow hydrodynamics.

Characteristics of instantaneous heat transfer rates in three heat-transfer-coefficient regimes

Instantaneous heat transfer coefficients (h) for flows of various fluids (mercury, air, water, and motor oil) over plates inclined at various angles have been fundamentally examined. We have discovered three distinctive regimes, in which fluids behave interestingly. In regime I, as h increases, instantaneous heat transfer rates (q) decrease, defying our physical intuition. Regime II exhibits maxima in q-versus-h curves. In regime III, h becomes a strong function of time especially for t∈[0,1s], revealing the possible deficiency of traditional lumped-capacitance models. Results of computational simulations are obtained using COMSOL commercial package whose algorithm is based on the Galerkin finite element method, and exhibit qualitatively-correct trends when compared with classical solutions for steady-state cases.

Investigation of single-droplet/wall collision heat transfer characteristics using infrared thermometry

This study experimentally investigated the heat transfer characteristics during the collision of a single droplet with a plate heated to temperatures ranging from 176 to 375°C. The heated plate was made of infrared (IR)-transparent sapphire with a very thin IR-opaque platinum film on top, which allowed the top-surface temperature to be measured from below using an IR camera. The dynamics of the droplets and the corresponding top-surface temperature distribution of the heated plate during collision were acquired using synchronized high-speed video and IR cameras. The three-dimensional transient conduction equation for the heated plate was solved numerically using the measured surface temperatures as boundary conditions, and the time- and space-resolved surface heat flux was obtained. Various physical characteristics associated with the heat transfer during the collision of a single droplet with a heated plate were analyzed, including the local heat flux distribution, effective heat transfer area, instantaneous heat transfer rate, total heat transfer, and vapor film thickness. In addition, the crucial surface temperature at which the total heat transfer from a single-droplet collision is significantly degraded, known as the dynamic Leidenfrost point, was detected.

A Numerical Study on the Convective Heat Transfer Characteristics of Pulsed Impingement Drying

Impinging jets issuing from the tailpipe of pulse combustors have been evaluated in recent studies for possible applications in rapid drying of continuous sheets such as grades of paper, textiles, etc. In order to further understand the effect of pulsed flows on the heat and mass transfer rates of impinging jets, a numerical study was performed on a two-dimensional pulsating impinging jet array. A computational fluid dynamics approach was used to examine the effect of periodic sinusoidal pulsation on the local Nusselt number distribution of the wet target surface being dried. Because a high temperature and large temperature difference between the jet flow and impingement surface are used to obtain high heat transfer rates in impingement drying, the thermophysical properties of jet flows were taken into account in the present mathematical model. A parametric study including phase angle and frequency as well as amplitude of pulsating flows was conducted for optimization and design of pulsating jet arrays. Examination of the velocity and thermal fields showed that the instantaneous heat transfer rate on the target surface was highly dependent on the mass transfer characteristic and development of the hydrodynamic boundary layer with time.

A study on the heat and mass transfer properties of multiple pulsating impinging jets

In order to explore the potential effect of unsteady intermittent pulsations on the heat and mass transfer rate of multiple impinging jets, a numerical study is performed on a two-dimensional pulsating impinging jet array under large temperature differences between jet flows and impingement wall when the thermo-physical properties can change significantly in the flow domain. Computational fluid dynamic approach is used to simulate the flow and thermal fields of multiple pulsating impinging jets. The numerical results indicate a significant heat transfer enhancement due to intermittent pulsation over a wide range of conditions. The oscillatory flow periodically alters the flow patterns in contrast to steady jets, which can eliminate the formation of a static stagnation point and enhance the local Nusselt number along the impingement wall between adjacent jets. Examination of the velocity field shows that the instantaneous heat transfer rate on the target surface is highly dependent on the hydrodynamic and thermal boundary layer development with time.

Turbulent impinging jet heat transfer enhancement due to intermittent pulsation

A numerical study was carried out of heat transfer under a pulsating turbulent slot impinging jet. The jet velocity was varied in an intermittent (on–off) fashion. The effects of the time-mean jet Reynolds number, temperature difference between the jet flow and the impinging surface, nozzle-to-target distance as well as the frequency on heat and mass transfer were examined. The numerical results indicate significant heat transfer enhancement due to intermittent pulsation of the jet flow over a wide range of conditions for both cooling and heating cases. Simulations of the flow and temperature fields show that the instantaneous heat transfer rate on the target surface is highly dependent on the hydrodynamic and thermal boundary layer development with time.

Generalized dynamic modeling of local heat transfer in bubble columns

Instantaneous local heat transfer rates were measured by using a hot-wire probe in three bubble columns of different diameters of 200, 400 and 800 mm. The time series of heat transfer rates were analyzed by means of rescaled range ( R/ S) and deterministic chaos analyses. Due to the influence of highly chaotic bubble motions, the instantaneous local heat transfer exhibits low-dimensional chaotic features. The dependences of Hurst exponents and Kolmogorov entropy on the column scale consistently suggest different nonlinear hydrodynamic behaviors exist in bubble columns of different scales. Based on the measurement of instantaneous heat transfer rates, an artificial neural network (ANN) was applied to correlate instantaneous local heat transfer with dynamic motions of bubble and liquid. The ANN was optimized and trained by only using the experimental data measured at one location of 200 mm column. The trained ANN model shows good performance for the generalized use to predict the dynamic heat transfer rate in three columns over whole experimental conditions studied, indicating the ANN is capable of capturing the universal relation between instantaneous heat transfer and local bubble dynamics.

Generalized dynamic modeling of local heat transfer in bubble columns

Instantaneous local heat transfer rates were measured by using a hot-wire probe in three bubble columns of different diameters of 200, 400 and 800 mm. The time series of heat transfer rates were analyzed by means of rescaled range (R/S) and deterministic chaos analyses. Due to the influence of highly chaotic bubble motions, the instantaneous local heat transfer exhibits low-dimensional chaotic features. The dependences of Hurst exponents and Kolmogorov entropy on the column scale consistently suggest different nonlinear hydrodynamic behaviors exist in bubble columns of different scales. Based on the measurement of instantaneous heat transfer rates, an artificial neural network (ANN) was applied to correlate instantaneous local heat transfer with dynamic motions of bubble and liquid. The ANN was optimized and trained by only using the experimental data measured at one location of 200 mm column. The trained ANN model shows good performance for the generalized use to predict the dynamic heat transfer rate in three columns over whole experimental conditions studied, indicating the ANN is capable of capturing the universal relation between instantaneous heat transfer and local bubble dynamics.

ENCLOSED BUOYANT CONVECTION OF A VARIABLE-VISCOSITY FLUID UNDER TIME-PERIODIC THERMAL FORCING

Finite-volume numerical solutions are obtained for buoyant convection of a fluid with temperature-dependent viscosity in an enclosed space. At one vertical wall the temperature is constant, and at the other vertical wall the temperature is time-periodic. Solutions to the governing Navier-Stokes equations are acquired for a fixed Rayleigh number and over wide ranges of viscosity contrast, which measures the ratio of viscosities at the cold side wall and hot side wall. The effects of variable viscosity on the time-mean value as well as the amplitude of fluctuation of instantaneous heat transfer rate are delineated. The extensive results reveal the existence of resonance and show that resonance becomes more distinctive for large viscosity contrast in the case of the hot-wall temperature oscillation. As the viscosity contrast increases, the upward convective motion is invigorated during the relative heating phase due to the lowered viscosity in the thermal boundary layer near the hot vertical side wall. This is also reflected in the augmentation of the cycle-averaged heat transfer rate. When the temperature oscillation is imposed on the cold wall, the flow is less sensitive to the viscosity variation. Physical interpretations of the overall flow and heat transfer are offered.

Local and instantaneous distribution of heat transfer rates through wavy films

Heat transfer through a laminar-wavy falling silicon oil film on a vertical plate has been investigated. The film flows down an electrically heated metal foil which delivers a constant heat flux. The temperature field at the backside of the foil is measured by a very sensitive infrared camera with high temporal resolution. Local and instantaneous heat transfer rates through the film are evaluated from the temporal development of the local wall temperature. Investigations in two-dimensional waves show the influence of the Prandtl number on the transport processes even in the laminar region. The experimental data confirm the results of numerical calculations with a spectral element method. Furthermore, the temporal averaged heat transfer has been investigated in thermally developed three-dimensional wavy films over a film Reynolds number range of 10–129 and a Prandtl number range of 10–45. The Prandtl number dependency of the heat transfer in the laminar-wavy region of the film agrees well with a recently published experimental study.

Scale-up effects on the time-averaged and dynamic behavior in bubble column reactors

Scale-up effects on the dynamic and time-averaged behavior are investigated in bubble columns with diameters of 200, 400 and 800 mm , respectively. Using a hot-wire probe, the local instantaneous heat transfer rates and the local time-averaged gas holdups were measured at the different locations of the columns over the gas velocity range from 20 to 90 mm/ s . The dyanamic behavior of local heat transfer was characterized by frequency domain and state-space analysis in terms of power spectrum, correlation dimension and Kolmogorov entropy. The power spectra of heat transfer rate fluctuations were found to be insensitive to the bubble column size. The state-space analysis indicates that heat transfer rate exhibits a chaotic behavior with the correlation dimension varying from 2.0 to 3.0. No significant effect of column size on the correlation dimension was observed, whereas the Kolmogorov entropies are apparently dependent on the column diameter. With the increase in column diameter, Kolmogorov entropy decreases significantly and exhibits rather flat radial distribution in the large-scale column. With respect to the time-averaged behavior of local gas holdup, the effects of column size on the radial distribution of gas holdup was examined. It was found that the 800 mm column exhibited a relatively uniform distribution of gas holdup. The dependence of both dynamic and time-averaged behavior on the column scale is considered to be closely related to the different macroscopic flow structure, which is a function of the scale of the column.