Sbaity, Ahmad AlamirLouahlia, HasnaLe Masson, Stephane
15页
查看更多>>摘要：This article presents experimental and analytical studies of thermosyphon loop with hybrid condenser for cooling data center under various climatic constraints. Two cooling modes are investigated: the passive cooling mode using only the thermosyphon loop without electrical consumption and the hybrid cooling mode using the pumped sea water to cool the condenser of the thermosyphon. Experiments are conducted under various operating conditions to show the influence of the selected parameters on the cooling capacity. Several cooling systems are compared: two different condensers, phase change materials are added in the form of internal plates fixed on the interior walls of the data center and finally the cooling mode of the condenser is also changed. The results show that the passive thermosyphon loop cooling capacity is 1750 W for ambient temperature at 23 degrees C and it increased to 2900 W at 10 degrees C. The cooling capacity is extended to 3000 W by using the thermosyphon with the condenser cooled by the sea water at 23 degrees C. The thermosyphon loop with a hybrid cooling condenser is investigated annually as the cooling system of a chosen typical data center in seven French coastal cities. The results show that the passive thermosyphon loop is generally more powerful for cooling data center in winter. In summer, the use of the sea water cooling of the condenser is required. Taking into account the climatic constraints, the annual energy saving can reach 54% using the combination of the passive and the hybrid thermosyphon condenser.
查看更多>>摘要：With the development of high-performance multi-core microprocessors, the increase in hotspots density poses a severe challenge to chip thermal management. Embedded cooling can improve the cooling performance at the chip level. However, the cooling efficiency will be deteriorated by the interaction of hotspots. This paper proposes the concept of thermal superposition effect based on the relationship between the cooling efficiency of two hotspots and their relative positions. The effective cooling area and superposition area describe the thermal characteristics and interaction of hotspots, and the superposition factor beta evaluates the intensity of interaction. By optimizing the superposition factor, the average temperature reduces 22%. Moreover, the factor can predict the average temperature of hotspots within the error under14%. Therefore, the thermal superposition effect and factor beta is an effective analysis tool in multi-hotspot cooling, simplifying the analysis process and making it possible to predict and optimize the interaction by experiment and finite element simulation.
查看更多>>摘要：Phase change material (PCM) has great potential in thermal control of aircraft electronic components because of their excellent latent heat capacity. In the current work, a finned copper foam phase change energy storage unit (PCESU) was fabricated using n-eicosane, 97.2% porosity copper foam and 0.8 mm fins. The effects of four different heating power i.e. 40 W, 45 W, 50 W, 55 W corresponding to heat flux of 0.4 W/cm(2 ) 0.45 W/cm(2 ) 0.5 W/cm(2), 0.55 W/cm(2) at four different centrifugal acceleration magnitudes, i.e. 0 g, 5 g, 9 g, 13 g with three different acceleration directions on the thermal performance of PCESU were experimentally studied in a systematic manner. Experimental results indicated that: (1) the acceleration direction has a significant effect on the thermal performance of PCESU which can be improved for the cases of vertical and opposite directions, whereas restrained for the case of same direction. Under acceleration condition, the average melting time for the cases of opposite and same direction are 15.19% and 37.10% longer than that for the case of vertical direction, respectively. The temperature difference of PCESU while the melting is completed is 95.12% higher than that for the case of vertical direction on average. (2) the effect of acceleration magnitude on the heat transfer performance can be determined significantly when the acceleration direction is applied. The melting time decreases with the increase of the acceleration magnitude along vertical direction and increases along same direction. The temperature difference decreases with the increase of the acceleration magnitude along vertical direction, whereas increases along opposite or same direction. Moreover, the melting time and temperature difference for acceleration magnitude changing from 0 g to 5 g have an obvious larger change rate than that from 5 g to 9 g and 9 g to13 g. (3) the melting time is negatively correlated to the heating power, whereas the temperature difference is positively correlated to the heating power. The proposed two-dimensional (2D) simplified model can be helpful to reveal the physical mechanism of the thermal performance of PCESU.
Han, ZhenxingWickramaratne, ChaturaGoswami, D. YogiJotshi, Chand...
14页
查看更多>>摘要：Inorganic salts are potential phase-change materials for medium- and high-temperature thermal applications. It is essential to acquire knowledge of their behavior in latent heat thermal energy storage for the design of storage devices and the construction of an energy conversion and utilization system. In this study, a eutectic mixture of NaNO3 (mass ratio of 46%) and KNO3 was selected as phase change material, and a nitrate salt-based latent heat thermal energy storage unit was built to experimentally investigate its operating characteristics during charging and discharging. The thermophysical properties of the eutectic nitrate salt were measured and presented. The air outlet temperature remained almost unchanged during the melting of salt in the charging process, but it decreased gradually during the discharging process because the thermal resistance increased with the salt solidification. The melting time was shortened by 31.0% and 38.1% when the air inlet temperature was increased from 260 degrees C to 270 and 280 degrees C, respectively. The solidification time was shortened by 22.2% and 33.3% when the air inlet temperature was reduced from 210 degrees C to 200 and 190 degrees C, respectively. When the air mass flow rate was increased from 0.964 g/s to 1.446 and 1.962 g/s, the melting time was shortened by 32.4% and 57.4%, respectively, while the solidification was only shortened by 8% and 16%, respectively. The charging ratio or discharging ratio can be calculated through heat-loss evaluation to depict the thermal energy change in a charging or discharging process. The results indicated that charging ratio increased almost linearly with the melting of the salt. The air mass flow rate had a significant impact on this parameter. The influence of the air inlet temperature was gradually weakened with the increasing air inlet temperature. Owing to the coupling effect of heat loss and airflow, the influence of air parameters on the discharging ratio was weak. The effectiveness of storage indicates the extent to which the latent heat of salt can be utilized. It increased from 29.7% to 52.8% when the air inlet temperature was reduced from 210 to 190 degrees C. This study provides insights into the phase-change characteristics of the nitrate salt and the nitrate salt-based latent heat thermal energy storage unit.
查看更多>>摘要：Targeting at decreasing the pressure drop in the micro heat sink, a gradient distribution pin fin arrays and narrow slots (GPN) is proposed. In addition, in order to further reduce the pressure drop of the micro heat sink, three improved are proposed, including GPN48, GPN2244, GPN264 by changed the number and position distribution of the pin fin arrays in the narrow slots. The results show that the pressure drop of the three improved gradient distribution pin fin arrays and narrow slots decreases by 28.7%-33.4% at a certain volumetric flow rate (Qv = 120 ml/min, Re = 563) when comparing with the conventional gradient distribution. Furthermore, when Re ranged from 188 to 1125, the maximum wall temperature of the three improved gradient distribution pin fin arrays and narrow slots decreases by 4.4 K-26.6 K when comparing with the gradient distribution (GPN). When the Re = 563, the heat flux limit of GPN is only 82 W/cm(2), while GPN48, GPN2244 and GPN264 reached 109 W/cm(2), 114 W/cm(2) and 120 W/cm(2) respectively. Especially, comparing with GPN, the average heat transfer coefficient of GPN48, GPN2244 and GPN264 increases by 11.9%-23.0% (Re = 118), which greatly improves the application reliability in the micro heat sink.
查看更多>>摘要：Throttle low-temperature closed-cycle Joule-Thomson refrigerators with low performance (compressor volumetric flow rate less than 25 m3/h) are one of the promising directions of development of refrigeration equipment. These refrigerators operate with mixtures of refrigerants and are capable of thermostatting different samples at the temperature below -160 degrees C. Such refrigerators have several special features so that it would take an integrated approach to design new types of refrigeration equipment based on Joule-Thomson refrigerators. This paper proposes such a method for mixtures with more than three components. The method comprises a design calculation stage and an experimental stage; the stages are interlinked. As a part of considering these design stages, we describe an original method for calculating the multi-component mixed refrigerant charge and the original algorithm of solving the problem of ensuring the "estimated circulated composition" in the steadystate operation mode in 3-5 iterations. Also, we present the results of experimental verification of the integrated approach to the design for the case of a low-temperature medical refrigerator created based on the original patented layout.
查看更多>>摘要：Falling film evaporation is a promising technology widely applied in refrigeration, desalination and beyond applications owing to the great advantages. However, the liquid film hydrodynamics and heat transfer performance are studied insufficiently. Compered with experiments, numerical simulations are economical and efficient especially in the study of liquid film, through which some microscopic understandings are expected to be extracted. In this paper, a comprehensive review for a large pool of computational studies about falling liquid film flow and heat transfer on the horizontal tube and tube bundle is presented. Computational methods of liquid-gas two-phase flow and mass transfer model, as well as relevant researches were first introduced. Then, the studies on the falling film hydrodynamics, film thickness, sensible heat transfer, flow pattern, evaporation and boiling outside the single tube, tube bundle, special-shaped tube and entire evaporator studied with 2D and 3D models are respectively reviewed in order. Then, the investigations on the falling film breakout and dryout are reviewed. And then, the numerical predictions of falling film thickness and heat transfer coefficient are involved. Afterthat, the benchmark data for falling film numerical simulation are summarized. Finally, some future needs and recommendation relating to crucial technologies that must be solved are proposed.
查看更多>>摘要：Adsorption cooling is a sustainable technology, since it can utilize solar energy or waste heat, while employing substances without ozone depletion and global warming potential. The adsorption reactor design is determinant for the system performance. An underexplored geometry hitherto - the hexagonal honeycomb adsorption reactor - was numerically investigated. An in-house, validated, three-dimensional computational model based on unstructured meshes was employed. The Specific Cooling Power (SCP) and Coefficient of Performance (COP) were quantified for several geometrical and operational parameters. The cell inradius creates a dichotomy between SCP and COP, being 218.9 W/kg(s) and 0.356 for 1 mm, while being 80.4 W/kg(s) and 0.606 for 6 mm. The cell height influences prominently the SCP, being 159.5 W/kg(s) and 86.1 W/kg(s) for 5 mm and 30mm, respectively. The fin thickness impacts mostly the COP, being 0.599 and 0.364 for 0.5 mm and 3 mm, respectively. Higher COP is achieved for higher evaporator, lower adsorption and lower condenser temperatures. Higher SCP is achieved for lower adsorption and condenser, and higher evaporator and desorption temperatures. Shorter cycles result in high SCP and low COP, whereas the inverse occurs for longer cycles. Aluminum heat exchanger yields 7.7% higher COP than copper. The results are discussed from a physical, as well as, an engineering perspective.
查看更多>>摘要：This paper focusses on the numerical investigation of a concrete thermal energy storage (CTES) system using air as heat transfer fluid (HTF). To reduce the number of simulations and treat complex interactions between parameters, the response surface models for multiple responses are established based on 27 specific design points which are determined by central composite rotation design (CCRD). With the response surface models, the effects of the CTES system's design parameters on its performance are analyzed. The results indicate that the HTF velocity is the most important factor affecting the charging time and charging energy efficiency. The HTF inlet temperature substantially influences the energy storage. The interactions also have a crucial influence on the performance indices. The optimization is carried out to minimize the charging time, and maximize the energy storage and the charging energy efficiency simultaneously. The optimal parameter combination is optimized by the desirability function. The discharging process is further considered, so that the overall performance of the cycle process is optimal. The geometrical parameter of 22 tubes and 4 fins is considered appropriate under the selected operating conditions. The proposed method provides an efficient means to efficient design of CTES unit.
查看更多>>摘要：Measurement uncertainty has significant negative impacts on the operation and control of heating, ventilation and air conditioning systems. It is a big challenge and should be solved urgently. Existing studies focus on reducing the impacts of measurement uncertainty by developing uncertainty tolerant methods without quantifying the measurement uncertainties themselves. They therefore fail to fundamentally solve them. This study aims to directly quantify the measurement uncertainties of water flow meters in multiple water-cooled chiller systems using a Bayesian approach. A measurement uncertainty quantification strategy is proposed based on Bayesian inference and energy balance models, and the Markov chain Monto Carlo method is used to achieve the strategy. The site data collected from a chiller system are used to test the strategy. Four simulation tests with different levels of measurement uncertainty are conducted to further test and systematically validate the strategy. Test results show that the measurement uncertainties (both systematic and random uncertainties) of the water flow meters in the chiller systems can be quantified effectively and with acceptable accuracy. The strategy performs very well in quantifying random uncertainties of flow meters, and the relative errors range from 0% to 12.8%. The performance of the strategy in quantifying systematic uncertainties is also satisfactory, and the relative errors range from 0.1% to 36.57%. The proposed strategy is able to quantify measurement uncertainties and can be used to optimize the control of chiller systems and improve the reliability of chiller systems.
NSTL
Elsevier