Similar Heat Transfer Performance Research Articles

Efficient thermal management of high-power electronics via jet-enhanced HU-type manifold microchannel

To efficiently remove the increasingly higher heat fluxes from electronic chips, this numerical study presents an embedded single-phase water-cooled heat sink which consists of a microchannel layer and a jet-enhanced HU-type manifold layer. The manifold features two liquid inlet plenums symmetrically positioned on two opposite sides and some outlets arranged in the middle. In order to reduce the average thermal resistance, the fluid impinging velocity is boosted by narrowing the manifold channel inlet. However, narrower inlet also leads to flow maldistribution which increases the maximum thermal resistance. To address this problem, we demonstrate two approaches: either adding a flat plate at the manifold outlet or adjusting the manifold channel walls to form a tapered pathway. Further, the impact of the plenum and manifold channel inlet profiles on fluid flow and heat transfer is investigated. Negligible difference is found between rectangular and trapezoidal plenums. As to the manifold channel inlet, although similar heat transfer performance is obtained with different shapes, the total pressure drop can be substantially different. To reduce the large pressure drop at the inlet, a semicircular or triangular inlet profile is preferred, instead of a rectangular one. Quantitatively, we show that for a 3 × 3 mm2 heated area, our heat sink can dissipate fluxes up to 2000 W cm−2 with a maximum temperature rise of 65 K and a pressure drop below 40 kPa, which represents one of the highest performances and energy efficiencies reported to date.

Heat Transfer Performance of Uni-Directional and Bi-Directional Lid-Driven Cavities Using Nanoparticle Enhanced Ionic Liquids (NEILS)

Two-dimensional numerical investigation has been carried out using an in-house CFD code to obtain the heat transfer performance of normal, uni-directional and bi-directional lid-driven cavity. The 2D in-house code is based on single phase approach for studying steady state laminar flows. Pure water (PW), ionic liquid (IL): 1-ethyl-3-methylimidazolium methanesulfonate; [C2mim][CH3SO3], ionic liquid mixtures (PW + IL) and alumina (Al2O3) nanoparticle enhanced ionic liquids (NEILS) are used in the present investigation. The thermophysical properties of all the heat transfer fluids used in the present numerical investigation were obtained from the experimentally measured data available in the literature. The obtained results showed that uni-directional lid-driven cavity exhibits 68.78 % higher Nuavg value at Re = 1900 in comparison with normal lid-driven cavity for pure water. The use of [C2mim][CH3SO3] ionic liquid also enhances the heat transfer performance by exhibiting 115.48 % higher Nuavg value in comparison with pure water at Re = 1900. Moreover, 15 % wt Al2O3 nanoparticles in 0.25PW + 0.75IL gives 21.2 % and 161.16 % higher heat transfer performance in comparison with ionic liquid and pure water respectively at Re = 1900. Furthermore, it is also observed that 10 % wt Al2O3 nanoparticles in 0.25PW + 0.75IL shows similar heat transfer performance like ionic liquid at all Re.

Pool boiling heat transfer and bubble behavior on the treelike networks with wedge-shaped channels

This paper designs the treelike networks with the wedge-shaped channel from the idea of bionics. Pool boiling heat transfer experiments of FC-72 under different liquid subcoolings were conducted on the networks with different branch levels. The bubble phenomenon was recorded to reveal the boiling mechanism. The bubble pumpless movement towards the diverging direction in the wedge-shaped channel under the action of capillary force was observed. Under the saturated condition, the surfaces are easier to be completely covered by bubbles and the effect of treelike networks is hindered, which results in similar heat transfer performance. With the increase of liquid subcoolings, the bubble fraction decreases and the effect of the branch level comes into effect. On the surface with high branch level, the bubbles are transported immediately at small radius by the narrow wedge-shaped channel. The moving bubbles disturb the fluid and coalesce with other bubbles, which increases the heat transfer coefficient. There are also more liquid supply pathways between small bubbles, which increases the critical heat flux. The importance of this research is to provide a solution to the difficulty of bubbles vertical departure caused by the lack of buoyancy under microgravity.

Pillow plate heat exchanger weld shape optimization using approximation and parallel parameterized CFD and non-uniform rational B-splines

Plate heat exchangers (PHXs) are compact with small approach temperature, and desirable thermal-hydraulic characteristics. It is greatly desirable to create novel PHX designs that utilize less material and less volume while attaining similar heat transfer performance and thus contribute significantly to energy conservation while lowering the environmental impact as well. Pillow Plate Heat Exchanger (PPHX) is a type of PHX with a complex wavy structure with desirable thermal-hydraulic characteristics, but yet an economical manufacturing process especially when compared to corrugated Chevron PHX (CPHX) which requires a special die for each design. PPHX has great design flexibility allowing novel designs to be created in a simpler and less costly process which provides them with a good potential to outperform other types of PHXs. PPHXs are more commonly used in chemical and process industry with limited research in Heating, Ventilating, Air Conditioning, and Refrigeration (HVAC&R) applications. In this study, the optimization of PPHXs in order to maximize their thermal-hydraulic performance, including weld shape optimization using novel weld shapes created using Non-Uniform Rational B-Splines (NURBS), is studied. Sensitivity analysis is conducted on the optimal designs to provide insights into factors affecting their performance. The potential enhancement is up to 36% improvement in heat transfer coefficient and 67% reduction in pressure drop as compared to a selected PPHX baseline design with circular spot welds. The optimal designs are compared to optimal CPHX designs from literature showing a potential improvement of 38% for heat transfer coefficient, and a reduction of 72% in pressure drop.

A liquid plug moving in an annular pipe – Heat transfer analysis

Different from a fully developed continuous flow in an annular pipe, a liquid plug moving in an annular pipe generates two toroidal vortexes, which cause radial transport that enhances heat transfer. The fully developed heat transfer of the concentric plug is studied for three types of thermal wall conditions: inner-flux, outer-flux, and isothermal. The fully developed heat transfer of the continuous flow is analytically solved for the same thermal wall conditions. The comparison of heat transfer is made between the plug and the continuous flow. Two heat transfer mechanisms, boundary layer transport and radial transport, are considered to explain the heat transfer enhancement. If the radial transport is weak, the plug shows similar heat transfer performance to the continuous flow, which is dominated by the boundary layer transport. The heat transfer enhancement relies on the radial transport, which increases with decreasing the inner radius, decreasing the plug length, and increasing the Peclet number.

Effect of tube shape on the condensation patterns of R1234ze(E) in horizontal mini-channels

In this paper, the condensation characteristic of R1234ze(E) in horizontal circular, square and triangle channels were numerically investigated. The detailed liquid-vapor interfaces, local film thickness and stream traces were presented for better understanding the condensation process in circular and non-circular tubes. Under the same hydraulic diameter, the void fraction in square and triangle tubes was lower than that in circular tubes at the same vapor quality. The square tube exhibited higher heat transfer coefficients than that of the circular tube, while the triangle tube showed similar heat transfer performance with the circular tube. The tube shape had no significant effects on the pressure gradients. The gravity played a dominant role in local film distribution in the circular tube, but was almost negligible in the square tube. Both the surface tension and gravity played an important role in film distribution in the triangle tube. The number and structure of vortexes in the square and triangle tube were very different from that in the circular tube. The tube shape has a significant effect on the liquid phase velocity at different circumference positions. Under the same external perimeter, either the heat transfer coefficients or the pressure gradients of the square or triangle tube were larger than that of the circular tube. The numerical data of heat transfer coefficients and pressure gradients were also compared with the predicted values from the available well-known correlations. The results of this paper are important for the industrial applications of alternative refrigerants.

Thermo-Fluidic Comparison between Sandwich Panels with Tetrahedral Lattice Cores Fabricated by Casting and Metal Sheet Folding

This numerical study compares single-phase forced convective heat transfer between two sandwich panels with tetrahedral metallic lattice cores separately fabricated by investment casting and the more cost-effective metal sheet folding method. The numerical model is validated against available experimental data. For a given Reynolds number and core porosity, the results reveal that the brazed sandwich outperforms the casted sandwich, exhibiting a 13% to 16% higher Nusselt number. Bigger vertexes and more evident blockage of mainstream by the ligaments are found to intensify the horseshoe vortex and the counter-rotating vortex pair upstream and downstream of each vertex. Relative to the casted sandwich panel, therefore, endwall heat transfer is enhanced by 22% to 27%, while similar heat transfer is achieved on the ligaments. It is also found that, for a given Reynolds number, the brazed sandwich induces a 1.6 to 1.7 times higher pressure drop relative to the casted sandwich due to more severe flow separation caused by the sharp edges of the rectangular ligaments. Finally, for a given pumping power, both sandwiches provide a similar heat transfer performance. Given that the brazed sandwich is more cost-effective and easier to fabricate than the casted one, the former may be superior from an engineering application point of view.

Revisit on natural convection from vertical isothermal plate arrays--effects of extra plume buoyancy

In this study, natural convection from vertical isothermal parallel-plate arrays is examined using 2-D steady-state numerical analysis. Extended computation domains encompassing the single- or multi-channel plate arrays are adopted. Also investigated are the consequences of using computation domains without the inlet extension or/and the outlet extension. The plate height is fixed at 100 mm, the plate spacing is 7 mm, and the plate thickness is 1 mm. The Elenbaas Rayleigh number Ra’ is fixed at 62.8. The results show that setting the inlet boundary at the entrance of the channel would ignore the separation formed near the outer entrance corner and the associated flow resistance; while setting the outlet boundary at the exit of the channel would omit the extra buoyancy provided by the hot plume above the arrays. In multi-channel arrays, the average heat transfer coefficients in different individual channels (hs) are different from each other. The h is the highest in the central channel and the lowest in the edge channel. Except for the edge channels, the h values are higher than that of a single channel. The differences in h between the central and the edge channels increase with increasing number of channels. This phenomenon can be ascribed to (1) the stronger extra hot-plume buoyancy in the inner plume region and (2) the higher entrance-separation resistance in the outer channels. The overall convection heat transfer coefficients in the plate arrays increase with increasing number of plate channels. The conventional assumption that all the channels of a multi-plate array have similar heat transfer performance needs re-evaluation.

Experimental study of falling film evaporation heat transfer on superhydrophilic horizontal-tubes at low spray density

This paper presents a series of experiments concerning heat transfer in a vertical row of horizontal-tube falling film evaporator. By visualization and heat transfer experiments, the effects of hydrophilic modification on the inter-tube flow patterns and evaporation heat transfer performance on different tubes, the superhydrophilic, hydrophilic and plain tubes, were investigated. The visualization results indicated that four flow patterns, droplet, droplet/jet, jet and jet/sheet mode was achieved in this study and the hydrophilic modification has little effect on flow mode transition. Experiments have been indicated that the superhydrophilic and hydrophilic tubes were completely wetted while the plain tubes were partially wetted at low spray density. The heat transfer results demonstrated that superhydrophilic tubes could maintain a high heat transfer coefficient at a wide range of flow rates (Res from 60 to 700). While, high enhancement was achieved at low spray density, at which the plain tube was not be wetted. When Res reached 350, three surfaces had the similar heat transfer performance. Simultaneously, the heating water inlet temperature had little effect on heat transfer coefficient for superhydrophilic tubes and had a negative influence on plain tubes and hydrophilic tubes at low spray density.

Polymer heat exchanger design for condensing boiler applications

Condensing boilers achieve very high efficiency levels by recovering both sensible heat and water vapor latent heat from the flue gas. Research since the 1980's has focused on corrosion in such condensing heat exchangers related to the acidic condensate and material selection. Polymers in condensing heat exchangers have been considered to avoid the cost and corrosion concerns of metallic designs. Past efforts have shown that polymers offer the advantage of corrosion resistance and cost, however, lower thermal conductivity limited their application. More recent developments have introduced thermally conductive polymers which now offer promising conductivity values. This project focused on the evaluation of a thermally conductive polymer heat exchanger for this application.Computational fluid dynamic results indicated thermal conductivity values of stainless steel, a typical heat exchanger material, do not need to be achieved for similar heat transfer performance. An increase in thermal conductivity from about 10 times that of the base polymer can achieve an overall heat exchanger effectiveness similar to that achieved with stainless steel. A polymer composite thermal conductivity of approximately 2.5 W/m⋅K would be adequate. Thermally conductive polymer materials are now commercially available which offer values up to 20 W/m⋅K. In this work, one Nylon-12 and one thermally conductive polymer composite heat exchanger prototypes were constructed for a condensing boiler application. Tests demonstrated that good overall heat transfer performance was achieved. The lower thermal conductivity of the polymer heat exchanger will lead to higher surface temperatures and lower water condensation rates.

Experimental studies on heat transfer of oxy-coal combustion in a large-scale laboratory furnace

A number of issues arise in the transition from air-firing to oxy-firing. In situations where the furnace should be retrofitted for oxy-firing, a similar heat transfer performance is crucial for operation of power plants. A survey of recent literature turned up very little detailed research on the characterization of pulverized oxy-coal flames generated by staged feed-gas burners and the impacts of burner settings on heat transfer performance. A large number of studies applied either O2 fraction upstream of the burner or recycle ratio as main parameter to characterize oxy-fired conditions. For this reason, this study applies other characteristic parameters, as for example swirl number and feed gas flow ratio between the burner registers, rather than only O2 concentration for operation of the test facility. Experiments were conducted in a test facility with a rated capacity of 0.40 MWth fired by an industry-type burner. A theoretical study was also carried out to calculate combustion parameters, e.g., adiabatic flame temperature, in order to define burner settings and assess the experimental results. The burner was set to operate at three levels of secondary swirl number (1.15, 1.65, and 2.05), while part of the feed gas was divided between the secondary and tertiary registers using flow ratios between 0.40 and 2.00. For completeness, three levels of O2 fraction upstream of the burner (29, 31, and 33 vol% O2) were also tested under oxy-fired conditions. Measurements of peak flame temperature, absorbed heat flux at the water-cooled walls, and total radiative heat flux were performed and applied besides heat balance calculations to evaluate heat transfer performance. Experimental data indicated the feed gas distribution among the burner registers control the heat transfer in the furnace in parallel with the swirl strength of the secondary flow for both air-fired and oxy-fired conditions. Even though lower flame temperatures were obtained in oxy-firing, the results suggest the possibility of matching similar heat uptake in the furnace to that found under air-fired conditions. Experimental data also indicated the choice of an O2 fraction of 31 vol% upstream of the burner, which enables a very close adiabatic flame temperature as air-firing, is an appropriated parameter for maintaining similar heat transfer conditions in the furnace.

Experimental investigation of Cu-based, double-layered, microchannel heat exchangers

Cu-based, single- and double-layered, microchannel heat exchangers (MHEs) were fabricated and assembled. Comparative measurements on liquid flow characteristics and heat transfer performance were conducted on these devices. Results were compared at the individual microchannel level as well as at the device level. The present results demonstrate that double-layered MHEs exhibit similar heat transfer performance while suffering a much lower pressure drop penalty compared to single-layered MHEs. Another Cu-based, double-layered, liquid–liquid counter-flow MHE was fabricated, assembled and tested. Results show that a low-volume, multilayered, high-performance, liquid-to-liquid MHE is achievable following the manufacturing protocols of the present double-layered, liquid–liquid counter-flow MHE.

Experimental Investigation of Uneven-Turn Water and Acetone Oscillating Heat Pipes

An experimental investigation of a new three dimensional oscillating heat pipe (OHP) with uneven turn design was conducted in order to develop a gravity-independent OHP. Different from other OHPs, the OHPs investigated herein have uneven turns. In the evaporator, the total turn number is 20 and in the condenser 14, i.e., 14 full turns and 6 half turns, which were constructed using copper tubing with an inner diameter of 1.65 mm. The water OHP was filled with water at a 62.5% filling ratio and the acetone OHP filled with acetone at a 62.7% filling ratio. Experimental results show that the uneven turn OHP developed herein can start the oscillating motion in the negative vertical position, i.e., the evaporator being above the condenser and demonstrate that the uneventurn OHPs can significantly reduce the gravity effect on the heat transport capability in an OHP. In addition, it is found that although at low power inputs the uneven turn OHP in the negative vertical position is not good as that in the positive vertical position (the evaporator being below the condenser), at high power inputs the uneven OHP in negative vertical position has similar heat transfer performance as in the positive vertical position. The uneven turn OHP achieved a thermal resistance of 0.135 °C/W with an operating temperature of 36 °C in the negative vertical position.

Heat Transfer Augmentation: Radiative-Convective Heat Transfer in a Tube With Fiber Array Inserts

Gas-phase heat transfer plays a critical role in many high temperature applications, such as preheaters, combustors, and other thermal equipment. In such cases common heat transfer augmentation methods rely on the convective component alone to achieve improved internal performance. Radiatively assisted heat transfer augmentation has been suggested as a way to overcome limitations in convective-only enhancement. One example of such a technique is the fiber array insert; thermal radiation emitted by tube walls is captured by a large number of slender fibers, which in turn convect heat to the flowing fluid. Previous numerical studies have indicated that this technique represents a promising enhancement method warranting further investigation. This paper presents results from an experimentally based feasibility study of fiber array inserts for heat transfer augmentation in an externally heated duct. Fibers composed of 140 μm silicon carbide and 150 μm stainless steel were assembled in arrays with porosities around 0.98, and were tested for empty-tube Reynolds numbers ranging from 17,500 to 112,500 and wall temperatures from ambient up to 750°C. The arrays cause a significant pressure drop—roughly two orders of magnitude higher than the empty-tube case—but tube-side heat transfer coefficients were improved by up to 100% over the convective-only case in the low flow rate regime. The stainless steel fiber array exhibited similar heat transfer performance as the silicon carbide case, although pressure drop characteristics differed owing to variations in fluid-structure flow phenomena. Pressure drop data were roughly within the range of d’Arcy law predictions for both arrays, and deviations could be explained by inhomogeneities in fiber-to-fiber spacing. Heat transfer was found to depend nonlinearly on wall temperature and flow rate, in contrast to previously reported numerical data.

Critical Heat Flux for Downward Facing Boiling on a Coated Hemispherical Surface

An experimental study was performed to investigate the effect of surface coating on the critical heat flux for downward facing boiling on the outer surface of a hemispherical vessel. Steady-state boiling experiments were conducted in the subscale boundary layer boiling (SBLB) facility using test vessels with metallic microporous coatings to obtain the local boiling curves and the local critical heat flux (CHF) limits. Similar heat transfer performance was observed for microporous aluminum and microporous copper coatings. When compared to the corresponding data without coatings, the boiling curves for the coated vessels were found to shift upward and to the right. This meant that the CHF limit was higher with surface coating and that the minimum film boiling temperatures were located at higher wall superheats. In particular, the microporous coatings were found to enhance the local CHF values appreciably at all angular locations explored in the experiments. Results of the present study showed that the microporous aluminum coating was very durable. Even after many cycles of steady state boiling, the vessel coating remained rather intact, with no apparent changes in color or structure. Although similar heat transfer performance was observed for microporous copper coatings, the latter were found to be much less durable and tended to degrade after several cycles of boiling.

Similarity and the formation of correlations for forced convection to supercritical pressure fluids

Investigation of heat transfer to supercritical pressure helium seems to be following a pattern like that of supercritical pressure water. The aim of the paper is to demonstrate the conditions under which heated tubes carrying variable property fluid will have similar heat transfer performance. It is shown how these conditions follow from the governing equations, and their application to the planning and interpretation of experiments is demonstrated using supercritical pressure water data. Recent experimental data and correlations for supercritical pressure helium are examined.