Longitudinal Heat Research Articles

Observation of boiling heat transfer and crisis phenomena in falling water film at transient heating

This paper deals with the experimental study on transient heat transfer at boiling and development of crisis phenomena in the falling film of subcooled liquid at stepwise heat generation. Experimental data with a high temporal and spatial resolution were obtained with the use of high-speed infrared thermometry and high-speed video recording synchronized simultaneously. It is shown that at vapor bubble appearance and condensation the convective streams are formed there, which lead to a decrease in temperature in the area behind the active nucleation site and local increase in heat transfer intensity. At the same time on the free surface of liquid film the so-called “temperature waves” propagating along the liquid flow are formed. The boiling curves were obtained on the basis of experimental data, and comparison with the known models for description of heat transfer at pool boiling of saturated liquid was performed. It is shown that mechanism of appearance and development of crisis phenomena depended significantly on the Reynolds numbers. Crisis development at high Reynolds numbers relates to dry spot formation at quasi steady-state boiling and their following expansion by the mechanism of longitudinal heat conductivity in the heating wall. The wave development on the free surface at low Reynolds number (Re=601), studied in current research, leads to the breakdown of the film without nucleate boiling due to the action of thermocapillary forces. The critical heat fluxes under these conditions are significantly lower than the critical values obtained for high Reynolds numbers at boiling in the falling films of subcooled water.

3D CFD analysis of the effect of inlet air flow maldistribution on the fluid flow and heat transfer performances of plate-fin-and-tube laminar heat exchangers

Plate fin-and-tube heat exchangers are widely used in several areas such as heating, ventilating, air conditioning and refrigeration systems. Nonuniformity of the inlet air flow in heat exchangers is first order importance and has decisive influence on their efficiency because it can intensify longitudinal wall heat conduction and the maldistribution of interior temperature.This study presents the results of three-dimensional (3D) Computational Fluid Dynamics (CFD) simulations aim to investigate the effect of inlet air flow maldistribution on the thermo-hydraulic performance of heat exchangers. Validation of the computation results with experimental data found in the literature has shown a very good agreement.Different computation test cases with a variety of inlet air flow distributions on in-line and staggered plate-fin-and-tube heat exchangers were run to systematically analyse their effects on system performance. The CFD results confirmed the importance of the influence of inlet fluid flow nonuniformity on heat exchanger efficiency. Results indicate that up to 50% improvement or deterioration in the Colburn j-factor are found compared to the baseline case of a heat exchanger with a uniform inlet air velocity profile. Moreover, the present investigation with respect to inlet flow maldistribution demonstrates that 3D CFD simulation is a useful tool for analysing, designing and optimising heat exchangers. Finally, the results of this study can be also of significant value for the optimum design of header and distributor configurations of heat exchangers to minimise maldistribution.

Sub-Kelvin Thermal Conductivity and Radioactivity of Some Useful Materials in Low Background Cryogenic Experiments

We present measurements of the thermal conductivity between 0.05 and 1 K, and radioactive contamination levels, for some thermally isolating materials. TIMET Ti 15-3-3-3, Mersen grade 2020 graphite, Vespel SP-1, Vespel SP-22, Vespel SCP-5000, Vespel SCP-5050, Graphlite CFRP, and a Kapton/epoxy composite are all investigated. Thermal conductivities were measured using a single-heater longitudinal heat flow method. Material radioactivity was determined for the materials at a low background counting facility using a high-purity gamma detector and GEANT4 Monte Carlo simulations.

Transient behavior of co-current parallel flow three-fluid heat exchanger

Transient behavior of co-current parallel flow three-fluid compact heat exchangers with the effect of two-dimensional longitudinal heat conduction through the separating sheet and axial dispersion in fluids has been investigated numerically by using the Gauss–Seidel iterative technique for step excitation provided to hot fluid inlet temperature. The results reveal that the performance of the heat exchanger is affected when two-dimensional longitudinal conduction in separating sheets and axial dispersion in fluids are considered.

Influence of cellular substructure on the thermal conductivity of heterovalent solid solutions of fluorides

The thermal conductivity of single-crystal samples of Sr0.88La0.09Nd0.03F2.03 and Sr0.80La0.15Nd0.05F2.20 solid solutions, both with a cellular substructure and without it, have been investigated by the method of stationary longitudinal heat flow in the temperature range of 50–300 K. An anisotropy of thermal conductivity (related to the substructure) is established for a Sr0.995Nd0.005F2.005 single crystal.

Heat spreading in a thin longitudinal fin

In view of the trend towards higher power densities in ever shrinking geometries, understanding heat spreading fundamentals is gaining importance. In this paper heat spreading in thin longitudinal geometries is considered. This geometry is of practical interest in one-dimensional Cartesian geometries. A characteristic length is derived and it is shown that this has physical significance for the distance that heat spreads, and for the total amount of heat cooled away. Furthermore, it is investigated when “thin” is a viable assumption. The use of the characteristic length is illustrated for the case of a line source cooling to a plate and for the case of the fins of a plate heatsink. The results are compared to numerical simulations. The work is an extension of the authors' earlier work on heat spreading in infinite longitudinal geometries and heat spreading in infinite and finite circular geometries.

Enhancement of heat transportation by oscillatory flow in a curved tube

The present paper deals with the longitudinal heat transportation by an oscillatory flow in a curved tube heat transportation pipe. The velocity and temperature fields during oscillatory water flow in a curved tube with inner diameter of 10mm were numerically simulated using the commercial software FLUENT. To clarify the effect of the curvature on heat transportation, the ratio of the tube radius to the curvature radius was varied from 0.0125 to 0.075, and oscillatory flow frequency was varied from 0.1 to 2.0Hz. The effective thermal diffusivity exhibited a peak at a curvature ratio around 0.05–0.07 and reached a value 12 times higher than that of a straight tube. It was also found that the dispersion of fluid particles due to secondary flow caused the enhancement of heat transportation.

500W 급 마이크로 가스터빈을 위한 파형 휜을 가지는 리큐퍼레이터의 열성능 비교

In this study, thermal performance of recuperators with plain and offset strip fins is investigated to enhance the thermal efficiency of a micro gas turbine. Thermal cycle analysis is conducted to determine major design parameters of a single-pass counterflow recuperator. In order to evaluate the performance of the recuperator, the effectiveness and the pressure drop in the recuperators are chosen as the objective function and the design constraint, respectively. The optimized geometries for internal structure of the recuperators with plain and offset strip fins are obtained with varying the fin spacing and height. From the result, the recuperator with offset strip fins has better thermal performance when the fin spacing, s, is smaller than 1.45mm and the thermal performance of the recuperator with plain rectangular fins is higher than that with offset strip fins in the region of <TEX>$s{\geq}1.45mm$</TEX>. In addition, it is found that the entrance region effect and the longitudinal wall heat conduction effect should be taken into account for accurately predicting the thermal performance of the recuperators with both plain and offset strip fins.

GAUSS QUADRATURES – THE KEYSTONE OF LATTICE BOLTZMANN MODELS

In this paper, we compare two families of Lattice Boltzmann (LB) models derived by means of Gauss quadratures in the momentum space. The first one is the HLB (N;Qx,Qy,Qz) family, derived by using the Cartesian coordinate system and the Gauss–Hermite quadrature. The second one is the SLB (N;K,L,M) family, derived by using the spherical coordinate system and the Gauss–Laguerre, as well as the Gauss–Legendre quadratures. These models order themselves according to the maximum order N of the moments of the equilibrium distribution function that are exactly recovered. Microfluidics effects (slip velocity, temperature jump, as well as the longitudinal heat flux that is not driven by a temperature gradient) are accurately captured during the simulation of Couette flow for Knudsen number (kn) up to 0.25.

Analytical solutions for transient temperature distribution in a geothermal reservoir due to cold water injection

An analytical solution to describe the transient temperature distribution in a geothermal reservoir in response to injection of cold water is presented. The reservoir is composed of a confined aquifer, sandwiched between rocks of different thermo-geological properties. The heat transport processes considered are advection, longitudinal conduction in the geothermal aquifer, and the conductive heat transfer to the underlying and overlying rocks of different geological properties. The one-dimensional heat transfer equation has been solved using the Laplace transform with the assumption of constant density and thermal properties of both rock and fluid. Two simple solutions are derived afterwards, first neglecting the longitudinal conductive heat transport and then heat transport to confining rocks. Results show that heat loss to the confining rock layers plays a vital role in slowing down the cooling of the reservoir. The influence of some parameters, e.g. the volumetric injection rate, the longitudinal thermal conductivity and the porosity of the porous media, on the transient heat transport phenomenon is judged by observing the variation of the transient temperature distribution with different values of the parameters. The effects of injection rate and thermal conductivity have been found to be profound on the results.

Neuronal differences between chronic low back pain and depression regarding long‐term habituation to pain

Longitudinal studies of experimental pain are rare and little is known about the differences regarding sensitization and habituation over longer periods in patients with chronic pain or depression compared with controls. We used a standardized longitudinal painful heat paradigm that was designed to induce long-term habituation in 19 patients with chronic low back pain (CLBP), 21 patients with depression (DEP) and 21 healthy participants (controls) over a time course of eight consecutive days. We applied functional magnetic resonance imaging on the first and last day of this period and after 3 months. Although the pain paradigm was standardized, patients with DEP exhibited significantly higher pain thresholds and a trend to higher pain ratings and, in functional imaging, showed less activation of the operculum and the secondary somatosensory cortex (S2) as compared to patients with CLBP and controls. Conversely, patients with CLBP showed increased activation in the anterior insula and parietal operculum as compared to patients with DEP and controls. Within session, all participants sensitized to pain, which was associated with higher activation levels in the thalamus, amygdala, midcingulate cortex, and sensory and motor areas. However, patients with depression showed significantly less activation in midbrain and brainstem areas. Given that pain and depression potentiate each other clinically, our data suggest that this may involve different cortical pain areas.

Simultaneous measurement of self-generated magnetic fields and electron heat transport in dense plasma

AbstractThe role of self generated magnetic fields in the transport of a heat wave following a nanosecond laser irradiation of a solid target is investigated. Magnetic fields are expected to localize the electron carrying the heat flux but at the same time are affected in their evolution by the heat flux itself. We performed simultaneous measurements of heat wave propagation velocity within the target and magnetic fields developing on the target surface. These were compared to results obtained by numerical magneto-hydrodynamic modeling, including self-generated B fields. The comparison shows that longitudinal heat flow is overestimated in the simulations. Similarly, but most notably, the radial expansion of the magnetic fields is underestimated by the modeling. The two are likely linked, the more pronounced radial drift of B-fields induces a rotation of heat flux in the radial direction, and corresponding longitudinal heat flux inhibition. This suggests the need for improving present modeling of self-generated magnetic fields evolution in high power laser-matter interaction.

접촉열전도재를 도포한 접촉열저항 특성연구

This paper describes the thermal contact resistance and its effect on the performance of thermal interface material. An ASTM D 5470 based apparatus is used to measure the thermal interface resistance. Bulk thermal conductivity of different interface material is measured and compared with manufacturers’ data. Also, the effect of grease void in the contact surface is investigated using the same apparatus. The flat type thermal interface tester is proposed and compared with conventional one to consider the effect of lateral heat flow. The results show that bulk thermal conductivity alone is not the basis to select the interface material because high bulk thermal conductivity interface material can have high thermal contact resistance, and that the center voiding affects the thermal interface resistance seriously. On the aspect of heat flow direction, thermal impedance of the lateral heat flow shows higher than that of the longitudinal heat flow by sixteen percent.

Numerical investigation of the thermohydraulic behaviour of a complete loop heat pipe

A complete steady-state model has been developed to determine the thermohydraulic behaviour of a loop heat pipe. The model combines a fine discretization of the condenser and the transport lines with a 2-D description of the evaporator. These original features enable to take into account heat losses to the ambient and through the transport lines as well as to evaluate the parasitic heat flux through the wick and the evaporator body. The present numerical simulations may improve the understanding of the physical mechanisms operating in an LHP evaporator, and their coupling with the other parts of the LHP, and provide guidance for the LHP design, aiming to reduce the thermal resistance of the system. The comparison between experimental data of a flat disk-shaped evaporator from the literature and theoretical simulations validates the proposed model. Simulations show the significance of heat conduction through the liquid line. Additionally, results show the major influence of the evaporation coefficient and of the wick thermal conductivity on the LHP operating temperature. When longitudinal heat losses are not significant, the competition between the parasitic heat flux through the wick and the heat transfer to the evaporation zone leads to an extremum for which the operating temperature is maximal. With a thermal conductive evaporator casing, the longitudinal parasitic heat flux strongly influences the LHP operational temperature and the evaporator energy balance.

Analytical solutions for movement of cold water thermal front in a heterogeneous geothermal reservoir

In the present study an analytical model has been presented to describe the transient temperature distribution and advancement of the thermal front generated due to the reinjection of heat depleted water in a heterogeneous geothermal reservoir. One dimensional heat transport equation in porous media with advection and longitudinal heat conduction has been solved analytically using Laplace transform technique in a semi infinite medium. The heterogeneity of the porous medium is expressed by the spatial variation of the flow velocity and the longitudinal effective thermal conductivity of the medium. A simpler solution is also derived afterwards neglecting the longitudinal conduction depending on the situation where the contribution to the transient heat transport phenomenon in the porous media is negligible. Solution for a homogeneous aquifer with constant values of the rock and fluid parameters is also derived with an aim to compare the results with that of the heterogeneous one. The effect of some of the parameters involved, on the transient heat transport phenomenon is assessed by observing the variation of the results with different magnitudes of those parameters. Results prove the heterogeneity of the medium, the flow velocity and the longitudinal conductivity to have great influence and porosity to have negligible effect on the transient temperature distribution.

Vertical Forced Vibration Effect on Natural Convective Performance of Longitudinal Fin Heat Sinks

This paper reports an experimental study for the effect of forced vertical vibrations on natural convection heat transfer, by the use of longitudinally finned plate made from Aluminum with dimensions (100, 300 and 3 mm). The sample of test was heated under the condition of a constant heat flux which is generated by applying an alternating voltage on a fixed resistor mounted on the underside of the sample test which was located horizontally or inclined in multiple angles at a range of (30°,60oand 90°). An experimental set-up was constructed and calibrated, 16 sets of fin-arrays were tested in atmosphere. Fin length, fin thicknesses and fin height were fixed at 300 mm, 3 mm and 13 mm respectively. The base-to-ambient temperature difference was also varied independently and systematically with the power supply to the heater ranging from 250 W/m2 to 1500W/m2. The frequency was varied at (0, 2, 6, 10 and 16 Hz) and the amplitude wain the range of (1.63-7.16 mm).The relation between the heat transfer coefficient and the amplitude of vibration is directly proportional with the inclination angle from (0o, 30o, 60o).Results show that the heat transfer coefficients of the heated plate in (30o) angle are about (19.27%) greater than those for the (60o) and exceeds that of the (90o) by (31.4%).

Longitudinal fluid flow and heat transfer between an elliptical hollow fiber membrane tube bank used for air humidification

The longitudinal fluid flow and heat transfer between an elliptical hollow fiber membrane tube bank (EHFMTB) used for air humidification are investigated. In the tube bank, the water stream flows inside the fibers, while the air stream flows axially between the fibers. The air and the water streams, in a counter flow arrangement, are exchanging heat and moisture effectively through the membranes. Two regularly populated arrangements of the tube bank, in-line and staggered, are considered. Two representative unit cells consisting of two fibers and the air stream flowing longitudinally between the fibers are selected as the calculation domains. The governing equations for the fluid flow and the heat transfer between the EHFMTB are established and numerically solved via a boundary-fitted coordinate transformation method. The friction factor and Nusselt number in the unit cells are then obtained, analyzed, and experimentally validated. They are compared with the data obtained for a hollow fiber membrane tube bank (HFMTB). The results are useful for structural optimization and performance evaluations of the EHFMTB employed for air humidification.

Effect of longitudinal wall conduction on the performance of a three-fluid cryogenic heat exchanger with three thermal communications

In addition to operating and design parameters, the thermal performance of heat exchangers, working in cryogenic temperature range, is strongly governed by losses such as longitudinal conduction through the wall, heat-in-leak from the surroundings, flow maldistribution, etc. In this paper, a three-fluid cryogenic heat exchanger, involving thermal interaction between all the three-fluids, is investigated for the effect of longitudinal heat conduction in the separating walls on its performance.

Comparative Study of Thermal Performance of Longitudinal and Transversal-Wavy Microchannel Heat Sinks for Electronic Cooling

Liquid cooling incorporating microchannels are used to cool electronic chips in order to remove more heat load. However, such microchannels are often designed to be straight with rectangular cross section. In this paper, on the basis of straight microchannels having rectangular cross section (SRC), longitudinal-wavy microchannel (LWC), and transversal microchannel (TWC) were designed, respectively, and then the corresponding laminar flow and heat transfer were investigated numerically. Among them, the channel wall of LWC undulates along the flow direction according to a sinusoidal function while the TWC undulates along the transversal direction. The numerical results show that for removing an identical heat load, the overall thermal resistance of the LWC is decreased with increasing inlet Reynolds number while the pressure drop is increased greatly, so that the overall thermal performance of LWC is inferior to that of SRC under the considered geometries. On the contrary, TWC has a great potential to reduce the pressure drop compared to SRC, especially for higher wave amplitudes at the same Reynolds number. Thus the overall thermal performance of TWC is superior to that of SRC. It is suggested that the TWC can be used to cool chips effectively with much smaller pressure drop penalty. In addition to the overall thermal resistance, other criteria of evaluation of the overall thermal performance, e.g., (Nu/Nu0)/(f/f0) and (Nu/Nu0)/(f/f0)1/3, are applied and some controversial results are obtained.

Thermal conductivity of FeS2 pyrite crystals in the temperature range 50–300 K

The thermal conductivity of two single-crystal samples of pyrite FeS2 are investigated by the method of stationary longitudinal heat flux in the temperature range 50–300 K. The low electrical conductivity of the crystals with a small impurity content causes an identical value of experimental lattice thermal conductivity. The temperature dependence of the phonon mean free path is established.