Ratio Of Heat Transfer Area Research Articles

An experimental method of determination and validation of latent thermal energy storage dynamic characteristic by effective enthalpy

The paper draws attention to the problems of evaluating stored heat capacity in a latent heat thermal energy storage (LHTES) that operates with unsteady inlet temperature conditions. These problems are studied for custom-built LHTES designed to be charged/discharged at varying heat transfer rates in a chilled water system. The LHTES incorporates a typical fin and tube heat exchanger and commercially available ATP 20 phase change material (PCM). For the designed storage construction, the ratio of heat transfer area to PCM volume is 354 m2/m3, and the ratio of PCM volume to the total volume (compactness factor) is 0.87. The LHTES thermal characteristics were investigated on a test stand using inlet temperature inputs changing linearly at 1, 2 or 3 K/h. During each test, a complete phase transition in the PCM was ensured. Based on LHTES inlet and outlet temperature profiles, enthalpy distributions versus outlet temperature were evaluated. The experiments proved that these distributions are not a material property as they depend on the geometric characteristics of the LHTES and charging/discharging rates. However, they can describe the thermal behaviour of the PCM filling the LHTES and include the influence of a non-uniform temperature distribution inside the LHTES on the PCM hysteresis and temperature range of the complete melting/solidification. These effects can be modelled by the proposed function of effective enthalpy. The effective enthalpy allows for relatively simple monitoring of phase transition progression in the LHTES based solely on the parameters measurable outside the storage.

Thermal performance improvement of a radial pulsating heat pipe with diverging channels by adopting Tesla valves at various heat fluxes

The thermal performance of a pulsating heat pipe (PHP) can be improved by increasing the flow circulation with the adoption of novel channel designs to minimize flow resistance. This study aimed to improve the thermal performance characteristics of a radial PHP with diverging channels by adopting Tesla valves. The effects of heat flux, condensation temperature, and heat transfer area ratio on the thermal resistance and heat transfer coefficient of the PHP with Tesla valves were measured under different heating and cooling conditions. In addition, the flow behaviors and pressure variations were analyzed for a short time frame. The PHP with Tesla valves exhibited a 17 %-lower thermal resistance than that without Tesla valves owing to the increased flow circulation with preferential movement in one direction. Furthermore, the heat transfer capability of the PHP with Tesla valves increased with increasing condensation temperature and heat transfer area ratio at the same heat flux. In addition, the optimal filling ratios were determined to be 70, 80, and 90 % for heat transfer area ratios of 0.13, 0.08, and 0.05, respectively. Finally, contour maps for the maximum temperature deviation were developed in terms of the heat flux, heat input, and heat transfer area ratio.

Experimental investigation and correlation analysis of pool boiling heat transfer on the array surfaces with micro-fins using FC-72 for the electronic thermal management

Pool boiling heat transfer characteristics were investigated on the both homogeneous and fractal array micro-pin-finned surfaces using FC-72 as working fluid. Two distinct fractal geometry pattern surfaces (Array4 and Array9) were designed to further explore their boiling heat transfer performance. The effect of different subcoolings (0 K, 15 K and 25 K) has also been analyzed combining fractal array surfaces. Bubble dynamics were captured using a high-speed camera. The results present the the array micro-pin-finned surfaces with less heat transfer area ratio can achieve similar or even higher CHF compared to the uniform micro-pin-finned surface (PF0.3–0.2–2), especially for subcooled boiling (ΔTsub = 25 K). Notably, bubbles nucleate and grow prior to reaching the micro-pin-finned unit region. This phenomenon is evident in visual images, indicating that the fractal array effectively impedes bubble coalescence, particularly during subcooled boiling. Besides, this fractal array surface can provide liquid supply channels and prevent dry-out. The mechanism behind the heat transfer enhancement and CHF triggering is elucidated based on liquid replenishment and bubble coalescence behaviors. A CHF prediction model was established considering the effect of liquid subcooling, rectangular array distribution, and micro-fins feature size. And the prediction results from this model show well agreement with experimental values and data points from literature, with mean absolute deviation being less than ± 15 %.

Data center energy efficiency enhancement potential of a membrane-assisted phase-change heat sink

This paper presents the latest progress on characterization of our membrane assisted phase-change heat sinks at conditions suitable for implementation in data centers. Experiments are conducted using water as the working fluid at a saturation pressure of 16 kPa, corresponding to a saturation temperature of ∼55 °C. This temperature is sufficiently lower than the silicon junction temperature of ∼80 °C. As anticipated, the overall performance of the membrane-assisted heat sink at sub-atmospheric pressure is lower compared to analogous tests at atmospheric pressure. In agreement with previous studies on membrane-assisted heat sinks, the critical heat flux limit increases with enhancement of the heat transfer area ratio and liquid space pressure. We report a maximum heat flux of 670 W/cm2 on a surface with enhanced area ratio of 3.45, multiple times greater than the heat fluxes reported hitherto by comparable two-phase heat sinks in literature. Heat transfer coefficients as high as 890 kW/m2-K are obtained. We compare the thermal performance of this device with other existing technologies including conventional microchannel heat sinks, thermosyphons, cold plates, jet impingement, and analyze its system level benefits.

Irreversibility of thermoelectric cooling model: Newtonian linear heat transfer and finite time thermodynamics

The irreversibility of the thermoelectric cooling (TEC) model could analyze and optimize refrigeration performance. In present work, a one-dimensional thermal conductivity model of the internal irreversible thermoelectric element was first established. Subsequently, two categories of evaluation indexes were further analyzed, i.e., cooling capacity Q c and coefficient of performance (COP). The thermodynamic model of TEC was then established jointly with finite time thermodynamics. The influences of current and the ratio of heat transfer areas between cold and hot sides on the steady-state characteristics of TEC were further discussed under Newtonian linear heat transfer. In addition, a transient thermodynamic model of TEC equipment was built to compare the effects of pulse width, pulse gradient, and pulse current on the cold side temperature of the TEC module. Theoretical results demonstrated that the effective working range of the steady-state operating current is 2A ≤ I ≤ 25A, and the area ratio of heat transfer between cold and hot sides is 0.07 ≤ x ≤ 1. The cooling capacity (Q c) and coefficient of performance (COP) could reach up to about 50W and 2.0, respectively. In addition, the increase of external heat transfer capacity of the hot side could reduce the transient supercooling temperature and the peak overshoot temperature, simultaneously. Afterward, the cooling capacity was remarkably enhanced. Current research could provide a reliable support for the optimization design and performance improvement of the thermoelectric cooler. Highlights Thermal conductivity model of internal irreversible thermoelectric element was established; Effects of cold and hot ends on the TEC were discussed under Newtonian linear heat transfer; Effects of pulse width, gradient and current on TEC cold end temperature were investigated; Heat transfer capacity of TEC hot end could reduce supercooling and overshoot temperatures; The irreversibility of TEC model will enhance its potentials for low grade energy exploitation.

Optimizing cooling electronic chips at high altitude with consideration of solar radiation

Maintaining a proper working environment for electronic chips is challenging for airships, as the ambient parameters at high altitude are largely different from those on the ground, which can influence the performance of cooling. This work aims to optimize the finned sink to minimize the weight with the consideration of the impact of solar radiation. By using a validated 3D model, it was found that the ignorance of the solar radiation can lead to a temperature deviation of 4.1 °C for electronic chips at 20 km when the solar radiation intensity was 1400 W/m2 and the wind speed was 10 m/s. Meanwhile, compared to the solar radiation intensity and the emissivity, the direction of solar radiation showed more impacts. In addition, even though the solar radiation doesn't influence the optimal fin height, fin number, and fin thickness, it would clearly affect the optimal heat transfer area ratio. As a result, it can clearly change the optimized weight, which was 5.7% higher if the solar radiation was not considered.

Investigation on heat transfer and pressure drop correlations of R600a air-cooled finned tube evaporator for fridge

The correlations to predict heat transfer coefficient and friction pressure drop of finned tube evaporator for fridge were developed. Twelve straight finned tube evaporators with different structural parameters were tested under different working conditions. The range of Reynolds number in air side was 750 < Rea < 9500 and that in tube side was 1000 < Rer < 15000. The models of evaporator heat transfer coefficient, air side pressure drop and tube side pressure drop were established respectively. The physical dimensions and flow parameters on both sides of finned tube evaporator were considered in the models and their effects on heat transfer and flow characteristics were revealed. The single-phase heat transfer area ratio was introduced. The correlations for household refrigerator evaporators were obtained through a nonlinear fitting method based on 210 groups of experimental data. The mean absolute percentage deviation of the new heat transfer correlation was 10.68%. The mean absolute percentage deviations of the new air side and refrigerant side pressure drop correlations were 8.8% and 10.3%, respectively. The data comparison showed that the proposed correlations were reasonable and accurate enough to be used in practical application.

Power and efficiency optimizations of an irreversible regenerative organic Rankine cycle

Rational and efficient uses of low temperature waste heats have become an important research topic, and organic Rankine cycle is one of effectual methods for the recoveries of waste heats. The performances of simple and regenerative organic Rankine cycles have been analyzed and optimized by using classical thermodynamics and finite thermodynamics. Based on finite time thermodynamics, an irreversible regenerative organic Rankine cycle model is built in this paper. With the fixed total heat transfer area of all heat exchangers (evaporator, condenser and regenerator) in the cycle, the power output and thermal efficiency of the cycle are maximized with the changes of heat transfer area ratios of the heat exchangers, mass flow rate of the working fluid and superheat. The influences of the internal irreversible losses on the power output and thermal efficiency of the cycle are analyzed. The results illustrate that there exist an optimal evaporator heat transfer area, an optimal mass flow rate and an optimal superheat to make the power output reach its triple maximum. There also exist an optimal evaporator heat transfer area ratio, an optimal regenerator heat transfer area ratio and an optimal superheat to make the thermal efficiency reach its triple maximum. The power output and thermal efficiency after triple optimizations are increased by 25.22% and 12.16%, respectively. Moreover, the internal irreversible losses have similar effects on the power output and thermal efficiency. Useful guidelines for the designs of the practical regenerative organic Rankine cycles can be offered from the gained results of this paper.

A parametric simulation on the effect of the rejected brine temperature on the performance of multieffect distillation with thermal vapour compression desalination process and its environmental impacts

AbstractMultieffect distillation with thermal vapour compression (MED–TVC) is one of the most attractive thermal desalination technologies for the production of freshwater. Several mathematical models were presented in the open literature to analyse the steady‐state performance of such process. However, these models have several limitations and assumptions. Therefore, there remains the challenge of having a reliable model to accurately predict the performance of the MED process. Thus, this research attempts to resolve this challenge by rectifying the shortcomings of the models found in the literature and create a new one. The robustness of the developed model is evaluated against the actual data of Umm Al‐Nar commercial plant situated in UAE. In seawater desalinisation, a large amount of high‐salinity stream (brine) is rejected back into the sea. This paper investigates the influence of the rejected (exit) brine temperature on the system performance parameters of MED–TVC process. Specifically, these parameters are considered as total heat consumption, gain output ratio, freshwater production, heat transfer area and performance ratio. Also, the particular parameters of TVC section of the entrainment ratio, compression ratio and expansion ratio are also addressed. Moreover, a critical evaluation of the influence of the rejected brine temperature on the seawater is also embedded.

Effectiveness of Fixed-Bed Regenerators for Energy Recovery in Buildings Applications

Energy consumption for ventilation purposes in buildings makes up a considerable portion of HVAC energy consumption. Energy recovery ventilators (ERVs) can reduce the required energy to pre-condition outdoor air in winter and summer seasons. Due to the high ratio of heat transfer area to volume, Fixed-Bed Regenerators (FBRs) can reach high effectiveness up to 90%. However, limited research studies are available for FBRs in HVAC applications. In this paper, a small-scale test facility is used to determine the sensible effectiveness of a FBR. Furthermore, a numerical model is proposed and validated against the experimental results from the small-scale test facility. The numerical results for latent effectiveness have been compared with available data in the literature and the comparison shows a satisfactory agreement between numerical and results from the literature.

Enhancing impurities removal from Si by controlling crystal growth in directional solidification refining with Al–Si alloy

In this paper, the principle of “plain materials” was attempted to advancing silicon (Si) purity, i.e., controlling solid-liquid interface and Si crystal growth pattern to enhance impurities removal from Si in directional solidification refining with Al–Si alloy. The relationship between Si crystal growth and impurity removal was established, and it was found that the removal ratio of metal impurities (less than 95% to more than 99%) gradually decreased with the order evolution of Si crystal from bulk Si, porous Si, acicular Si to Al–Si eutectic alloy. More than 99.5% of metallic impurities could be removed in the case of bulk silicon. However, the removal of boron (B) and phosphorus (P) showed the opposite trend. According to the principle of constitutional supercooling, the crystal Si growth and interface morphology were evaluated based on the conservation of mass of silicon atoms at the solidification interface, which was also used to explain the mechanism of impurities removal. Control of silicon crystal growth was achieved by changing the temperature gradient (lowering rate), Si alloy composition (Si content) and the heat transfer mode of solid-liquid interface that was expressed in terms of the effective heat transfer area ratio between wall and bottom of crucible (LA). A more stable and flat liquid-solid interface and 45% bulk Si was obtained at a lowering rate of 0.05 mm/min when the value of LA was 0.1, which can greatly enhance the removal of impurities.

Performance investigation of a two-phase transcritical CO2 ejector heat pump system

This study of the thermodynamic performance of a transcritical CO2 ejector heat pump cycle presents a numerical analysis under the constraint of constant total heat transfer area. An experimentally validated model of a CO2 heat pump system and a model of a two-phase ejector are combined to perform the study. The effects of chosen parameters on system performance are investigated: operating conditions as determined by the gas cooler and evaporator pressures; ejector design parameters including the primary throat diameter, effective area ratio, and diffuser outlet diameter; system design as defined by the heat transfer area ratio. The results of this analysis allow specifying the optimal range for the design parameters in order to maximize the COP and the heating capacity of the heat pump. The heat transfer area ratio is shown to have important effects on the COP and heating capacity as well as the optimal gas cooler pressure. As the heat transfer area ratio increases, COP and heating capacity increase as well, while the optimum gas cooler pressure decreases. At given ejector geometries and operating conditions, this optimization can increase the COP and heating capacity of the ejector heat pump by approximately 17% and 20% respectively by increasing the heat transfer area ratio.

Optimization of a separator assisted two-phase thermosyphon loop by using entropy generation analysis

In this paper, an optimization model is developed for a separator assisted two-phase thermosyphon loop and basic two-phase thermosyphon loop based on the entropy generation minimization method. The total entropy generation rate of the system is proposed as an optimization objective function and the total heat transfer area of evaporator and condenser is considered as a constraint. The model takes into account the irreversibility due to both irreversible heat transfer and pressure drop. The results show that there always exists minimum total entropy generation rate of the two systems when the heat transfer area ratio varies. The optimal heat transfer area ratios corresponding to the minimum total entropy generation rate also lead to the minimum total thermal resistance of the system and achieve the lowest temperature of the cooled object. Besides, the separator assisted two-phase thermosyphon loop can gain lower total entropy generation rate than the basic two-phase thermosyphon loop. Furthermore, the effects of the heat load, the total heat transfer area, the working fluid charge, the inner diameter of the separator and the height of two-phase tube on the minimum total entropy generation rate and the corresponding area ratio are also evaluated.

Performance evaluation and parametric analysis of AMTEC/TEG hybrid system

A general mathematical model of the AMTEC/TEG hybrid system consisting of alkali metal thermal electric converter (AMTEC) and thermoelectric generator (TEG) has been developed, in which the load parameters and main geometric parameters were considered. Instead of specifying some parameters, the evaporating temperature and condensing temperature of the AMTEC as well as the operating temperature of the TEG subsystem were determined by solving energy balance equations. Results show that electrode current density has a quite noticeable influence on both AMTEC subsystem and TEG subsystem. The maximum conversion efficiencies for AMTEC and AMTEC/TEG are 27.42% and 31.33%, respectively. Besides, it is interesting to note that the optimal value of the ratio of thermoelectric module area to electrode area is always near 0.2 regardless of the ratio of evaporator heat transfer area to electrode area. The length of thermoelectric arms l and the ratio of the thermoelectric couple area to the total area of the thermoelectric module b are critical factors, b ≥ 0.55 and l ≤ 11 mm should be satisfied to obtain a compact and low-cost system.

Experimental investigation on the thermal behavior of cylindrical battery with composite paraffin and fin structure

The thermal behavior of the cylindrical battery was examined with composite paraffin and fin structure through experimental measurements and benchmarking with other thermal management techniques. The mock-up cylindrical battery, made of aluminum, had vertical straight fins as enlarged heat transfer area to submerge in the paraffin wax as phase change material (PCM) with a maximum melting temperature of 44°C. For the pure PCM case, the melting process could be divided into AB segment with battery temperature ramp-up, BC segment with a clear-cut temperature plateau, and CE segment to reach complete melting, which is also basically represented in the evolutionary trend for the battery top to bottom temperature variation. The instantaneous Nusselt number would increase around the CE segment with a small portion of unmelted PCM as visualized by the numerical analysis, but the battery temperature would ramp up from the plateau in spite of the incomplete melt. Thermal enhancement with the PCM-fin cases was examined experimentally and the logarithmic dependence of the time-averaged Nusselt number is correlated with the heat transfer area ratio. In addition, the effective thermal control point C is found to relate to the melting front intersecting the bottom of the metal housing, and the corresponding thermal resistance is used to benchmark the thermal performance of PCM based thermal management systems. Independent of heating power, such a parameter can be correlated with the melting temperature to the ambient temperature difference for existing thermal management systems. It is found that the present composite PCM-fin system had the advantages of good thermal performance with prolonged work time.

Performance analysis of organic Rankine cycles using R600/R601a mixtures with liquid-separated condensation

Organic Rankine cycle (ORC) is a promising technology for power generation from medium to low temperature heat sources (<350°C). Although a zeotropic mixture can improve ORC efficiency, its large condenser heat transfer area is a crucial factor that impedes the broad application of ORCs using zeotropic mixtures. The liquid-separated condensation method is the process of separating the condensed fluid from the two-phase flow during condensation. This method has potential to effectively reduce the condenser heat transfer area of an ORC using a zeotropic mixture by increasing the condensation heat transfer coefficient and reducing the pressure drop. This study introduced the liquid-separated condensation into ORCs using R600/R601a mixtures. The effects of vapor quality at the liquid-separated unit inlet (xLSI) on the net power outputs, condensation heat transfer coefficients and heat transfer areas were analyzed. Results show that a xLSI below 0.3 maximizes the average condensation heat transfer coefficient for the R600/R601a mixture. The liquid-separated condensation can increase the average condensation heat transfer coefficient by 23.8% and reduce the condenser heat transfer area by 44.1% compared to the conventional condensation. Although the liquid-separated condensation may slightly reduce the maximized net power output for a low xLSI, the decrease of total heat transfer area is larger than that of the maximized net power output. Thus, the ratio of the total heat transfer area to the net power output can be reduced by 28.3%. The R600/R601a (0.9/0.1) mixture with xLSI=0.3 obtains the lowest electricity generation cost for the R600/R601a mixtures with single-stage liquid-separated condensation.

Experimental study of heat transfer characteristics of high-velocity small slot jet impingement boiling on nanoscale modification surfaces

Jet impingement boiling is one of the most efficient methods to increase the critical heat flux. Through controlling surface chemical properties and topography, plain nickel foil surface, three kinds of plain nickel-based chemical treatment surfaces and four kinds of nickel-based electrochemical treatment surfaces with nanocone array structure were used in the jet impingement boiling experiments to investigate the quantitative effects and the effect mechanism of the surface characteristic parameters, including nanoscale roughness (Ra): solid–liquid contact angle (CA) and effective heat transfer area ratio (r: the ratio of the actual heat transfer area to its projected area) on the heat transfer coefficient (HTC) and the critical heat flux (CHF). It is revealed that: similar to the heat transfer in pool boiling, changing nanoscale Ra has little effect on the heat transfer characteristics; decreasing solid–liquid CA can enhance the HTC while worsen the CHF obviously; increasing r can intensify both the HTC and the CHF. Predictive correlations of the HTC and the CHF of jet impingement boiling which include the effects of surface characteristics are presented. The predictive correlations presented can be widely used in various conditions with different working fluids and heating surfaces.

Multi-objective optimization of geometrical parameters of corrugated-undulated heat transfer surfaces

To achieve a maximum heat transfer capability and a minimum pumping power for corrugated-undulated (CU) heat transfer surfaces, a multi-objectives genetic algorithm was used to obtain the optimal values of the pitch and height of undulated plate (U-plate) and the height of the corrugated plate (C-plate) by using the Pareto optimal strategy. For this purpose, computational fluid dynamics (CFD) simulation, support vector machine (SVM) and the fast non-dominated sorting genetic algorithm were combined together and used for the optimization process. Three dimensional numerical simulations were performed to investigate the effect of geometrical parameters on the thermos-hydraulic performance of CU heat transfer surface. The maximum deviation for the Nusselt number and friction factor between the simulation and the published data were 8.81% and 13.1% respectively when the Reynolds number ranged from 1500 to 10,000. The flow and temperature profile in the CU passage were analyzed. Intensive secondary flows occurred in the C-plate channel and the U-plate channel due to the drag effect between the main flows in the two channels. And the effects of Reynolds number and structure parameters were studied. The change of U-plate height rather than that of U-plate pitch would have a dominant effect on the disturbance influence of U-plate. Besides, two SVM models were trained by the CFD results to predict the Nusselt number and friction factor of flow in CU passages with different geometrical and operational parameters. The comparison between the SVM predictions and the CFD results showed that the SVM models could predict the numerical data with a good accuracy. In addition, two evaluation criteria were proposed from perspectives of the manufacturers and the users, respectively. Finally, a set of optimized solutions were obtained. The optimal values of pumping power ratio and heat transfer area ratio between different CU passages and the standard one were in the range of 0.8–3.1 and 0.5–1.2, respectively. The manufacturers and the users can select the best design points according to their considerations.

Parametric optimization and performance analysis of ORC (organic Rankine cycle) for diesel engine waste heat recovery with a fin-and-tube evaporator

This paper presents the parametric optimization and performance analysis of ORC (organic Rankine cycle) system using GA (genetic algorithm) for recovering exhaust waste heat of a diesel engine. First, the effects of three key parameters, including evaporation pressure, superheat degree and condensation temperature on the system performances are conducted with POPA (net power output per unit heat transfer area) and EDR (exergy destruction rate) as objective functions. The optimal evaporation pressure, superheat degree and condensation temperature corresponding to the maximum POPA and the minimum EDR are provided under engine various operating conditions. Subsequently, as a key component of the ORC system, the performance of a fin-and-tube evaporator used in this ORC system is evaluated. The results show that the optimal evaporation pressures are mainly influenced by the engine operating conditions, whereas the superheat degree and the condensation temperature present a slight variation over the whole operating range. Furthermore, the PPTD (pinch point temperature difference) in the evaporator may occur at different positions with the variation of engine operating conditions. At engine maximum rated power point, the ORC system can achieve maximum POPA of 0.74 kW/m2, and the ratio of maximum effective heat transfer area to actual area of the evaporator is 69.19%.

Experimental study on the saturated pool boiling heat transfer on nano-scale modification surface

Plain nickel foil surface, three kinds of plain nickel-based chemical treatment surfaces and four kinds of nickel-based electrochemical treatment surfaces with nano-cone array structure were used in the pool boiling experiment. The quantitative effects and the effect mechanism of the surface characteristic parameters, including nano-scale roughness (Ra), solid–liquid contact angle (CA) and effective heat transfer area ratio (r: the ratio of the actual heat transfer area and its projected area) on the heat transfer coefficient (HTC) on the modification plain surfaces and nano-cone array surfaces were studied and summarized. By improving the previous proposed the HTC prediction correlation for the common plain surfaces (without considering the effect of r), a new uniform correlation is proposed and it can well predict the HTC of the heated surface with simple convex structure (with considering the effect of r). In addition, the bubble dynamic characteristics including the bubble departure diameter, the bubble departure frequency, the active bubble nucleation site density and the combined bubble were investigated by visualization research. These results proposed some mechanism support for the new predicting correlation.