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• New LTNE model considers thermal conduction apart from thermal convection. • Parameters for randomly-packed beds are derived from the results of pore-scale computations. • Regional considerations of the new analysis model better in line with physics. • Presence of the finite-thickness wall seriously affects temperature prediction. • Conventional methods fail to provide convincing predictions. A new analysis process for the LTNE (local thermal non-equilibrium) model to solve for steady-state, fully-developed convection in mono-sized-sphere packed bed is proposed to predict the temperature distribution inside a fixed bed. For an actual mono-sized-sphere packed bed, the voidage lies between those of the regularly arranged beds of FCC, BCC, and SC. Through an interpolation/extrapolation process based on the pore-scale computation results of the SC, BCC, and FCC packings, the heat transfer parameters of a randomly packed bed can be calculated. To reliably estimate the local heat transfer rate, the regional mass flow rates in a packed bed are accurately predicted first. Then, the regional solid-phase thermal conduction can be independently evaluated without interference from the regional fluid-solid convection heat transfer. Additionally, a finite-thickness wall is constructed into the computational model to significantly improve the accuracy of the predicted radial temperature profiles. With realistic temperature self-adjustment within the wall, there is no need to set unrealistic boundary conditions of either isothermal, isoflux or their combinations at the outer face of the porous zone, as many conventional methods in the literature do. The solution process and necessary parameters are summarized into flowcharts. Finally, the accuracy of the new LTNE analysis process in predicting the temperature distribution throughout the packed bed is validated with the literature experiment. In contrast, the conventional methods fail to provide the correct trend of the temperature variation. [ABSTRACT FROM AUTHOR]

Abstract This study numerically investigates the thermal performance of horizontal tube with circular fins under natural convection. The natural convection from horizontal finned-tube heat sink has been understood in an average sense since most previous studies were experiment-based. The present study reveals the flow and heat transfer characteristics in details with numerical analysis. The parameters include fin diameter (D), fin spacing (b), fin material, and tube diameter (d), under a fixed fin thickness of 0.5 mm and temperature difference of 50 K between the tube and the ambience. The distributions of fin temperature and heat transfer coefficients are calculated for stainless-steel (SS) and aluminum (Al) fins under various conditions. For Al fins with small fin temperature variations, decreasing tube diameter leads to higher heat transfer rates due to the larger fin surface areas; but for SS fins with large fin temperature variations, increasing tube diameter enlarges heat transfer rates. On large SS fins, negative local heat transfer coefficients may exist above the tube as the hot plume generated by the horizontal tube transfers heat back to the fin. A larger d leads to a slightly larger optimum fin spacing b opt. In addition, various existing empirical b opt formulae are examined for their reliability with respect to different D / d ratios and Al and SS fins, respectively with small and large fin temperature variations. Highlights • Aluminum and stainless-steel fins are examined to present different fin temperature variations. • Under small fin temperature variations, decreasing tube diameter increases heat transfer rate. • Under large fin temperature variations, increasing tube diameter increases heat transfer rate. • On large SS fins, negative local heat transfer coefficients may exist above the tube. • The optimum fin spacing is studied versus tube diameter and fin temperature variation. [ABSTRACT FROM AUTHOR]

• Inserting straight dividing board to 5.6 mm ID tubes incites pulsation in 3-D HPHX. • Water turns from thermosyphon to PHP mode under lower heat loads. • PHP mode observed for HFE-7000 or R134a with raised HX effectiveness. • HX effectiveness significantly raised for three fluids at different filling ratios. • The thermal performance enhancement can be attributed to three reasons. In our previous study, we have shown that a heat pipe heat exchanger system modified from a commercial air-to-air heat exchanger operates without pulsations for working fluids of dielectric HFE-7000 or R134a, or under low heat loads for water, due to the large internal tube diameter of 5.6 mm. To incite pulsation and enhance the thermal performance, a straight dividing copper board is inserted into the straight section of each of the tube to form two semicircular compartments with an effective hydraulic diameter of 3.4 mm. The heat transfer characteristics and thermal performance against the hot air inlet temperature for the novel boarded model and the non-boarded model are experimentally explored with three working fluids (water, HFE-7000 and R134a) under six filling ratios (35%, 38.5%, 50%, 55%, 65%, 71.5%). Because water already works in a PHP mode in the non-boarded model under high heat loads, the thermal performance in the boarded model improved mildly by 2.7%–10.6%. Whereas, for HFE-7000 and R134a, the heat transfer mode is successfully incited from thermosyphon in the non-boarded model into PHP in the boarded model. The most significant thermal performance improvement was observed for HFE-7000 by 8.3%–24.5%. The improvement can be attributed to the switch of heat transfer mode, the generated corners for condensate accumulation, and the additional heat conduction via the copper board. [ABSTRACT FROM AUTHOR]

• Novel heat pipe HXs made by modifying commercial 3-D Fin-and-Tube HXs were tested. • Different characteristics observed for three different fluids and charge ratios. • As tube ID=5.6 mm, water in PHP mode with stronger pulsation for larger filling ratio. • HFE-7000 or R134a in thermosyphon mode, but with fluctuating temperatures measured. • Three fluids in HX system with 3 HPHXs display similar maximum thermal performance. A novel, robust, high-performance, simple-constructed 3-D fin-and-tube HPHX is modified from a commercial product and designed for air-to-air waste heat recovery. Also, the heat transfer phenomena inside tubes and the thermal performance of the HPHX system have been experimentally explored with three kinds of working fluids (water, HFE-7000 and R134a) under three filling ratios (35%, 50%, 65%). At a tube ID of 5.6 mm, water works as a PHP, and the pulsation happened more easily with a larger filling ratio. Although HFE-7000 or R134a works in the thermosyphon mode, temperature instabilities were still measured under specific hot air inlet condition. This may be attributed to partial dry-out in the evaporator and condensed liquid or slug accumulation in the upper U bends of the condenser. As the performance of a single HPHX is considered, water outperforms the other two fluids, due to the successful actuation of pulsations. But when three HPHXs are installed in the heat exchange section, the maximum effectiveness and heat transfer rate of the system are 0.71 and 1466 W, respectively, roughly the same for the three working fluids. [ABSTRACT FROM AUTHOR]

Abstract: A parametric study on the dynamic natural convection from long horizontal fin arrays (L =128, 254 and 380mm) is made using a 3-D unsteady numerical analysis. The investigated range of height H is 6.4–38mm and that of spacing S is 6.4–20mm. Increasing L tends to enhance the oscillating sliding-chimney flow, whereas increasing S weakens it. The time-averaged overall convection heat transfer coefficients are larger for high and short fin arrays, due to the stronger buoyancy and thinner boundary layers. The –S relation exhibits a steep drop when S is narrowed below a threshold, which is larger for lower and longer fins. The optimum fin spacing S opt occurs near the threshold S, below which the benefit of increasing heat transfer area surrenders to the decrease of caused by excessive viscous drag. The heat transfer rate per unit base area Q/A b decreases with increasing L and decreasing H. The predicted dependence of S opt on H and L agrees well with experimental results and is explained based on numerical results of flow and heat transfer characteristics. The present predictions of Nu agree well with the correlations in the literature which use S as the characteristic length. [Copyright &y& Elsevier]

Abstract: The evaporation process in the wick of an operating flat-plate heat pipe was studied for three different working fluids having widely different figures of merit: deionized water, methanol and acetone. A sintered two-layer 100 + 200 mesh copper wick was adopted. Uniform heating was applied to the copper base plate near one end, and cooling with a water jacket at the other end. The evaporator resistance was determined using the temperature difference between the copper plate and the vapor respectively under and above the evaporator center. While the steady-state evaporation process was visualized through the upper glass plate of the heat pipe, the evaporator resistances were measured simultaneously. The maximum heat loads for water are far greater than those of methanol and acetone, with their values correlating well with their figures of merit. The minimum evaporator resistances for the three working fluids differ slightly in spite of the large differences in their properties. The differences can be mainly attributed to the different average thicknesses of the evaporating liquid layers, according to visualization. While nucleate boiling was not observed for water, very weak and localized nucleate boiling was observed for methanol near the maximum heat loads, beyond which local dryout began. For acetone, slow, periodic up-and-down motion of the liquid layer as a result of nucleate boiling was observed near the maximum heat loads over a significant portion of the evaporator. Beyond the maximum heat loads, nucleate boiling was again suppressed. [Copyright &y& Elsevier]

Abstract: The evaporation resistances of loosely-sintered copper-powder evaporators were measured in operating flat-plate heat pipes. The evaporation processes was also visualized through a top glass plate. Irregular or spherical powders of different size distributions were investigated. Uniform heating of 16–170W/cm2 was applied to the base plate near one end with a heated surface of 1.1×1.1cm2. At the other end was a cooling water jacket. The evaporation performance was first examined with the effect of liquid flow resistance minimized, i.e., the copper powders covered only the heated area with the remaining region covered with sintered copper wire screens. Similar to multi-layer mesh wicks, quiescent surface evaporation without nucleate boiling was observed for all test conditions, in spite of the abundant nucleation sites. The water film receded and the evaporation resistance reduced with increasing heat flux. Once partial dryout occurred, the evaporation resistance re-rose. The minimum evaporation resistances were about 0.08–0.09Wcm2/K for wicks containing fine powders. These values are similar with those for multi-layer-mesh wicks having a fine bottom screen. In the absence of fine powders, the minimum evaporation resistances were significantly larger. In the second part for homogeneous sintered-powder wicks, the large flow resistance tended to retard the condensed water from returning to the evaporator. However, this can be compensated by a larger charge and/or a thicker wick. [Copyright &y& Elsevier]

Abstract: This work presents visualization of the evaporation/boiling process and thermal measurements of operating horizontal transparent heat pipes. The heat pipes consisted of a two-layered copper mesh wick consisting of 100 and/or 200 mesh screens, a glass tube and water as the working fluid. Experimental results indicated that nucleate boiling was prompted for a wick having a fine 200-mesh bottom layer. When the fluid charge approximately equaled the pore volume in the wick, the water–vapor interface receded into more curved menisci with increasing heat load Q. Thus, larger capillary forces and evaporation areas were attained to meet the increasing need of liquid supply and evaporation rate at the water–vapor interface. At Q =40 and 45W, the water film became less than 100μm and the nucleate boiling observed at lower heat loads disappeared. Optimal thermal characteristics with smallest thermal resistances across the evaporator and lowest overall temperature distributions were found for such a wick/charge combination. Under a smaller charge, partial dry-out was observed in the evaporator. Under a larger charge, liquid recession with increasing heat load was limited and bubbles grew and burst violently at high heat loads. The effects of different wicks and fluid charges on the evaporation/boiling characteristics were discussed. [Copyright &y& Elsevier]

A pulsating heat pipe (PHP) contains a wickless design with aligned serpentine tube configuration whose simple structure offers a comparatively easy manufacturing capability. The bends with large curvature are often used for serpentine PHPs. This eventually results in a decline in effective contact surface area between evaporator/condenser and PHP circuitry, thereby impairing the benefit of the wickless design of a PHP. A novel thermal module featuring a 3-D configuration pulsating heat pipe, an evaporator, and a fin-and-tube condenser is proposed to tackle the high-flux application. Methanol is used as the working fluid with a filling ratio of around 60%. Test results indicate the thermal resistance of the proposed module varies from 0.148 K/W to 0.0595 K/W when the supplied power changes from 100 to 1000 W. The proposed thermal module can handle a supplied power up to 1 kW and the corresponding power or heat flux is much higher than any existing literatures. [ABSTRACT FROM AUTHOR]

Abstract: This work presents visualization and measurement of the evaporation resistance for operating flat-plate heat pipes with sintered multi-layer copper-mesh wick. A glass plate was adopted as the top wall for visualization. The multi-layer copper-mesh wick was sintered on the copper bottom plate. With different combinations of 100 and 200 mesh screens, the wick thickness ranged from 0.26mm to 0.8mm. Uniform heating was applied to the base plate near one end with a heated surface of 1.1 × 1.1cm2. At the other end was a cooling water jacket. At various water charges, the evaporation resistances were measured with evaporation behavior visualized for heat fluxes of 16–100W/cm2. Quiescent surface evaporation without nucleate boiling was observed for all test conditions. With heat flux increased, the water film receded and the evaporation resistance reduced. The minimum evaporation resistances were found when a thin water film was sustained in the bottom mesh layer. With heat flux further increased, partial dryout appeared with dry patches in the bottom mesh holes, first at the upstream end of the heated area and then expanded across the evaporator. The evaporation resistance re-rose in response to the appearance and expansion of partial dryout. When the fine 200 mesh screen was used as the bottom layer, its thinner thickness and stronger capillarity led to smaller minimum evaporation resistances. [Copyright &y& Elsevier]