The Experts below are selected from a list of 6021 Experts worldwide ranked by ideXlab platform
Richard Bonner - One of the best experts on this subject based on the ideXlab platform.
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vapor chambers with hydrophobic and biphilic Evaporators in moderate to high heat flux applications
Applied Thermal Engineering, 2018Co-Authors: Mohammad Reza Shaeri, Daniel Attinger, Richard BonnerAbstract:Abstract Two new types of vapor chambers were fabricated to investigate the influence of engineering the wettability of Evaporators for possibility of simultaneously reducing the thermal resistances and increasing the critical heat flux (CHF). One vapor chamber was fabricated by pressing a hydrophilic screen mesh wick structure into a hydrophobic evaporator through support posts integral to the condenser portion of the vapor chamber. The second vapor chamber included a biphilic evaporator without an evaporator wick, as such, the superhydrophilic zone surrounding the central hydrophobic area directly above the heater provided the capillary forces. Despite their lower thermal performances compared with those of the baseline vapor chamber fabricated by sintering the hydrophilic copper particles onto a hydrophilic copper substrate, both novel vapor chambers exhibited promising performances, as such, the vapor chamber with a hydrophobic evaporator approached CHF at high heat fluxes around 105 W/cm2, and the one with a biphilic evaporator did not approach the incipience of CHF until 162 W/cm2. The mechanical contact between the screen mesh and the hydrophobic evaporator, and the patterning of the biphilic evaporator were identified as the key roles in thermal performances of the tested vapor chambers. Suggestions are given to improve the thermal performances of the proposed vapor chambers in this study. If successful, the fabrication cost of these vapor chambers will be reduced by skipping high-temperature sintering process on the Evaporators, the method that is used to fabricate the commonly used sintered-powder vapor chambers.
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feasibility study of a vapor chamber with a hydrophobic evaporator substrate in high heat flux applications
International Communications in Heat and Mass Transfer, 2017Co-Authors: Mohammad Reza Shaeri, Daniel Attinger, Richard BonnerAbstract:Abstract A novel vapor chamber was fabricated to assess the feasibility of combining hydrophobic and hydrophilic wettabilities in the evaporator to optimize thermal performance. The proposed vapor chamber included a separate layer of hydrophilic sintered copper powder wick that was pressed in intimate contact with a hydrophobic evaporator substrate with a water contact angle around 140°. The contact between the wick layer and the evaporator was provided by sixteen posts implemented on the condenser, which pushed the wick layer toward the evaporator. The thermal performance was evaluated based on the thermal resistance, source temperature, and temperature uniformity across the condenser. Results were compared with those of a baseline vapor chamber that was fabricated by sintering hydrophilic copper particles on a hydrophilic copper evaporator substrate. The wick size and the copper powders used to fabricate the wick structure were the same in both vapor chambers. Overall, the performance of the proposed vapor chamber was lower than that of the baseline vapor chamber, possibly due to microscale gaps between the wick layer and the evaporator substrate. However, the concept of using a hydrophilic wick to force liquid in contact with a hydrophobic evaporating surface could enable a new family of vapor chambers with low thermal resistance, if more efficient techniques for improving the mechanical contact between the wick layer and the evaporator are introduced through further detailed research. If successful, the fabrication cost of vapor chambers would be reduced as well, by using prepared wick structures, which do not require high-temperature sintering processes on Evaporators.
Changqing Tian - One of the best experts on this subject based on the ideXlab platform.
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simulation on vertical microchannel evaporator for rack backdoor cooling of data center
Applied Thermal Engineering, 2020Co-Authors: Binfei Zhan, Shuangquan Shao, Hainan Zhang, Changqing TianAbstract:Abstract The rack-backdoor cooling technology used with a vertical microchannel evaporator of a loop thermosyphon system has a great application prospect in data center cooling. Thus, investigating the evaporator’s performance is necessary especially under inhomogeneous air-side conditions according to the actual operation scenarios of the rack. This study establishes a multiple heat source physical model of a 42U rack and the mathematical models of rack cooling on the basis of actual products. The entire heat transfer and temperature distribution are verified. After being heated by servers and before entering an evaporator, inhomogeneous air temperature and flow velocity data can be obtained with the physical rack model. The maximum difference is up to 6 °C and 0.6 m/s. The air temperature on top is 0.6 °C higher than that at the bottom of inside the rack due to the thermal lift in the vertical direction. The mathematical model indicates that the maximum difference of the refrigerant vapor quality among vertical tube outlets can reach 0.14, whereas the maximum air-side temperature difference of the evaporator outlet is 9.5 °C. These simulation results can provide effective guidance for the optimization design and application of vertical microchannel Evaporators.
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experimental investigation on a loop thermosyphon with three Evaporators unique startup and oscillation phenomena
International Journal of Refrigeration-revue Internationale Du Froid, 2019Co-Authors: Hainan Zhang, Shuangquan Shao, Changqing TianAbstract:Abstract Loop thermosyphon with multiple Evaporators has tremendous potential in applications that involve multiple heat sources. In this paper, a loop thermosyphon with three Evaporators is investigated experimentally. Unique phenomena of startup and oscillation compared with common loop thermosyphons are reported. In the startup process, a unique phenomenon called “sudden temperature change” is found. This phenomenon occurs in a temporary state in the startup, when the temperature differences between the Evaporators without heating power and with heating power reach certain range (−2.6 °C to 0.8 °C in this experiment). In the quasi-steady state, an oscillation map which shows the range when oscillation occurs is obtained. Compared with only one evaporator with heating power, simultaneous heating on multiple Evaporators reduces the oscillation frequency, enhances the oscillation amplitude and sometimes makes it irregular, due to the interaction between the Evaporators. Also, for every two adjacent Evaporators, the phases of oscillation are almost opposite.
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the transient response oscillation and internal flow of a loop thermosyphon with dual Evaporators
International Journal of Refrigeration-revue Internationale Du Froid, 2018Co-Authors: Hainan Zhang, Shuangquan Shao, Yuping Gao, Changqing TianAbstract:Abstract Loop thermosyphon with multiple Evaporators is an ideal equipment to achieve multi-source heat transfer with low energy consumption. In this paper, the transient response and oscillation characteristics of a loop thermosyphon with dual Evaporators under different heating conditions, as well as the internal flow and interaction between the Evaporators, are tested experimentally and visually. The results show that when the heating power is uniform, oscillation is more significant under higher heating power, which is due to the conflict and bypass flow between the Evaporators; when the heating power of one evaporator is higher, outlet flow of the other evaporator is impeded to some extent and bypass flow from this evaporator to the other exists periodically. As a special case, when only one evaporator has heating power input, this bypass flow remains relatively stable; when the thermosyphon works at steady state, the working state changes smoothly if the heating power changes as step function.
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the effect of heating power distribution on the startup time and overshoot of a loop thermosyphon with dual Evaporators
Applied Thermal Engineering, 2018Co-Authors: Hainan Zhang, Shuangquan Shao, Yuping Gao, Changqing TianAbstract:Abstract Loop thermosyphon with multiple Evaporators is a promising device in multi-source heat transfer. The startup performance is very important for its thermal control ability. In this paper, the effect of heating power distribution on the startup of a loop thermosyphon with dual Evaporators is investigated experimentally. The startup time and stationarity under different power distributions are analyzed utilizing three parameters: peak time, transition time and temperature (pressure) overshoot. The results show that the startup process is faster and the overshoot of pressure and temperature is larger when the distribution is more uneven; Heating on one evaporator with the same heating power with the other evaporator makes the startup process longer while it makes the overshoot smaller or even disappear; The transition time is approximately twice as much as the peak time when the peak time exists.
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experimental study on a dual parallel evaporator heat pump system for thermal management of electric vehicles
Energy Procedia, 2017Co-Authors: Guiying Zhang, Fei Qin, Huiming Zou, Changqing TianAbstract:Abstract A dual-parallel-evaporator heat pump system, which has two parallel Evaporators for cabinet air-conditioning and battery temperature control respectively, is designed to realize thermal management for electric vehicles. System performance and refrigerant mass flow distribution character of two branches at different opening degrees of electric expansive valve are tested. Changes of refrigerant superheat degree and pressure at the outlet of battery side evaporator caused by the decreasing of the battery temperature have great effect on mass flow rate of cabinet branch. The maximum mass flow rate reduced ratios of cabinet branch is 20.5% when the opening degree of electric expansive valve (EEV) of battery branch is 0.3. Stable performance of this system on rated condition is tested by adjusting the opening degree of two EEVs to get the optimal status. The experimental results indicate that integration operation can supply additional 1.04 kW cooling capacity for battery unit with very little compressor power increase. However, cooling capacity of cabinet side only decreases by 0.95%. The additional parallel evaporator is desirable to solve the battery cooling problem.
Claudio Melo - One of the best experts on this subject based on the ideXlab platform.
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effect of frost morphology on the thermal hydraulic performance of fan supplied tube fin Evaporators
Applied Thermal Engineering, 2017Co-Authors: Diogo Londero Da Silva, Claudio Melo, Christian J L HermesAbstract:In refrigeration applications, frost build-up on the evaporator surfaces depletes the thermal-hydraulic performance due to an increase in the thermal resistance and the blockage of the air flow. These negative effects depend not only on the total amount of frost, but also on the frost morphology that affects the density and, therefore, the thermal conductivity of the frost layer. In this paper, the studies of frost accretion on fan-supplied tube-fin Evaporators carried out in previous works are advanced to investigate the effects of the frost morphology on the thermal-hydraulic performance of fan-supplied Evaporators. A visual inspection of the frost deposition was carried out to characterize the frost morphologies under different operating conditions. Moreover, a mathematical model was used to figure out how the operating conditions and the geometric parameters affect the cooling capacity and the pressure drop. The visualization confirmed that the operating conditions have a strong effect on the in-situ frost morphology. In addition, the numerical results not only illustrate the impact of the frost morphology on the evaporator performance, but also point out that the number of defrost operations may be reduced if a proper heat exchanger design is adopted.
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in situ study of frosting and defrosting processes in tube fin Evaporators of household refrigerating appliances
International Journal of Refrigeration-revue Internationale Du Froid, 2011Co-Authors: Fernando T Knabben, Christian J L Hermes, Claudio MeloAbstract:Abstract The present paper advances an in-situ study of frosting and defrosting processes in tube-fin Evaporators of household refrigerators and freezers. Frost growth experiments were carried out using a purpose-built test facility comprised of a refrigerated cabinet placed inside a climatized chamber, and a vapor compression refrigeration loop that controls both the evaporating temperature and the evaporator superheating. A first-principles two-dimensional simulation model was put forward to predict the evaporator blocking over time, and also the heat and mass transfer rates. The model validation exercise has shown that the model follows well the experimental trends of pressure drop, frost mass, cooling capacity and air flow rate, with all predictions falling inside ±10% error bounds. The model was then used to assess the impact of several design parameters on both the evaporator blockage and defrost efficiency. It was found that a nearly ideal defrost efficiency is achieved when the defrosting process is performed by two simultaneous heaters.
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experimental study of frost accumulation on fan supplied tube fin Evaporators
Applied Thermal Engineering, 2011Co-Authors: Diogo Londero Da Silva, Christian J L Hermes, Claudio MeloAbstract:Compact tube-fin Evaporators have been extensively used in refrigeration cassettes for light commercial applications. Such refrigeration systems are space constrained and, therefore, the heat exchangers (condenser and evaporator) must have a large area-to-volume ratio. In addition, such applications require a subfreezing evaporating temperature that induces the growth of a frost layer on the finned surface, which may block the evaporator if a proper defrost strategy is not used. Before completely blocking the evaporator, the frost layer depletes the heat exchanger performance by adding an extra thermal resistance and also by reducing the fan-supplied air flow rate. Understanding the way the frost forms on these compact heat exchangers and also the way the fan is affected by frost clogging is mandatory for the design of robust refrigeration systems and also to devise more efficient defrost strategies. In this study an experimental investigation on the frost accretion of tube-fin Evaporators considering the fan characteristics is carried out. To this end, a specially designed, constructed and calibrated closed-loop wind-tunnel facility was used. Experimental tests were carried out with four different (three wavy-fin and one louvered-fin) evaporator coils under different conditions. It was found that the frost formation rate increases with the air flow rate, supercooling degree and the density of fins. A strict relation between accumulated mass of frost, air-side pressure drop and cooling capacity was also observed. It was also noted that the fan characteristics play an important role on the evaporator thermal performance, indicating that under frosting conditions the fan-evaporator pair must be designed as a coupled system. Furthermore, for the same operating conditions, the louvered-fin evaporator showed to be more sensitive to the frost formation effects than the wavy-fin coils.
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a study of the air side heat transfer and pressure drop characteristics of tube fin no frost Evaporators
Applied Energy, 2009Co-Authors: Jader R Barbosa, Claudio Melo, Christian J L Hermes, Paulo J WaltrichAbstract:A study is presented on the influence of the air flow rate and surface geometry on the thermal-hydraulic performance of commercial tube-fin ‘no-frost’ Evaporators. A specially constructed wind-tunnel calorimeter was used in the experiments from which data on the overall thermal conductance, pressure drop, Colburn j-factor and Darcy friction factor, f, were extracted. Eight different evaporator samples with distinct geometric characteristics, such as number of tube rows, number of fins and fin pitch were tested. Semi-empirical correlations for j and f are proposed in terms of the air-side Reynolds number and the finning factor. A discussion is presented on the performance of the Evaporators with respect to specific criteria such as the pumping power as a function of heat transfer capacity and the volume of material in each evaporator.
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air side heat transfer and pressure drop in tube fin accelerated flow heat exchangers
2007Co-Authors: Paulo J Waltrich, Jader R Barbosa, Claudio Melo, Christian J L Hermes, Joel Boeng, Daniel S Vieira, Rodrigo KremerAbstract:An experimental analysis of heat transfer and pressure drop on the air-side of tube-fin Evaporators utilized in household 'frost-free' refrigerators is carried out in the present paper. The performance of an alternative evaporator concept, the so-called Accelerated Flow Evaporator (AFE) is systematically investigated for the first time. In this evaporator, the air-side cross sectional area decreases with the distance from the air flow inlet causing the air flow to accelerate and promote an enhancement of the air-side heat transfer coefficient. This heat transfer enhancement allows a reduction of the heat exchanger volume and hence its material cost. Comparisons between the experimental results and Computational Fluid Dynamics (CFD) simulations are also presented, with the purpose of validating a numerical methodology to be used as a design tool for this new type of evaporator.
Rahmatollah Khodabandeh - One of the best experts on this subject based on the ideXlab platform.
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pressure drop in riser and evaporator in an advanced two phase thermosyphon loop
International Journal of Refrigeration-revue Internationale Du Froid, 2005Co-Authors: Rahmatollah KhodabandehAbstract:Abstract This study presents an experimental investigation of pressure drop in the Evaporators and the riser of an advanced thermosyphon loop. The thermosyphon was designed for the cooling of three parallel high heat flux electronic components. The tested Evaporators were made from small blocks of copper in which 7, 5, 4, 3, 2, 1 vertical channels with the diameters of 1.1, 1.5, 1.9, 2.5, 3.5 and 6 mm, respectively, and a length of 14.6 mm were drilled. Tests were done with isobutane at heat fluxes ranging between 22.4 and 303 kW/m 2 . For prediction of the pressure drop, in the riser, different combinations of frictional pressure drop and void fraction correlations were tested. Regarding the evaporator a simple correlation based on a homogeneous model [M.B. Bowers, I. Mudawar, Two-phase electronic cooling using mini-channel and macro-channel heat-sinks—part II, flow rate and pressure drop constraints, ASME J Electron Packaging 116 (1994) 298–305. [1] ] has been used to predict the pressure drop.
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heat transfer in the evaporator of an advanced two phase thermosyphon loop
International Journal of Refrigeration-revue Internationale Du Froid, 2005Co-Authors: Rahmatollah KhodabandehAbstract:Abstract As heat generation from electronic components increase and the limit of air-cooling is reached, the interest for using liquid cooling for high heat flux applications has risen. Thermosyphon cooling is an alternative liquid cooling technique, in which heat is transferred as heat of vaporization from evaporator to condenser with a relatively small temperature difference. The effect of fluid properties, the structure of wall surfaces, and the effect of system pressure was investigated and reported previously by the author. In this paper, the influence of heat flux, system pressure, mass flow rate, vapor fraction, diameter of evaporator channel and tubing distance between evaporator and condenser on the heat transfer coefficient of an advanced two-phase thermosyphon loop is reported. The tested Evaporators were made from small blocks of copper with 7, 5, 4, 3 and 2 vertical channels with the diameters of 1.1, 1.5, 1.9, 2.5, and 3.5 mm, respectively and the length of 14.6 mm. Tests were done with isobutane at heat fluxes ranging between 28.3 and 311.5 kW/m 2 .
Christian J L Hermes - One of the best experts on this subject based on the ideXlab platform.
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effect of frost morphology on the thermal hydraulic performance of fan supplied tube fin Evaporators
Applied Thermal Engineering, 2017Co-Authors: Diogo Londero Da Silva, Claudio Melo, Christian J L HermesAbstract:In refrigeration applications, frost build-up on the evaporator surfaces depletes the thermal-hydraulic performance due to an increase in the thermal resistance and the blockage of the air flow. These negative effects depend not only on the total amount of frost, but also on the frost morphology that affects the density and, therefore, the thermal conductivity of the frost layer. In this paper, the studies of frost accretion on fan-supplied tube-fin Evaporators carried out in previous works are advanced to investigate the effects of the frost morphology on the thermal-hydraulic performance of fan-supplied Evaporators. A visual inspection of the frost deposition was carried out to characterize the frost morphologies under different operating conditions. Moreover, a mathematical model was used to figure out how the operating conditions and the geometric parameters affect the cooling capacity and the pressure drop. The visualization confirmed that the operating conditions have a strong effect on the in-situ frost morphology. In addition, the numerical results not only illustrate the impact of the frost morphology on the evaporator performance, but also point out that the number of defrost operations may be reduced if a proper heat exchanger design is adopted.
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in situ study of frosting and defrosting processes in tube fin Evaporators of household refrigerating appliances
International Journal of Refrigeration-revue Internationale Du Froid, 2011Co-Authors: Fernando T Knabben, Christian J L Hermes, Claudio MeloAbstract:Abstract The present paper advances an in-situ study of frosting and defrosting processes in tube-fin Evaporators of household refrigerators and freezers. Frost growth experiments were carried out using a purpose-built test facility comprised of a refrigerated cabinet placed inside a climatized chamber, and a vapor compression refrigeration loop that controls both the evaporating temperature and the evaporator superheating. A first-principles two-dimensional simulation model was put forward to predict the evaporator blocking over time, and also the heat and mass transfer rates. The model validation exercise has shown that the model follows well the experimental trends of pressure drop, frost mass, cooling capacity and air flow rate, with all predictions falling inside ±10% error bounds. The model was then used to assess the impact of several design parameters on both the evaporator blockage and defrost efficiency. It was found that a nearly ideal defrost efficiency is achieved when the defrosting process is performed by two simultaneous heaters.
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experimental study of frost accumulation on fan supplied tube fin Evaporators
Applied Thermal Engineering, 2011Co-Authors: Diogo Londero Da Silva, Christian J L Hermes, Claudio MeloAbstract:Compact tube-fin Evaporators have been extensively used in refrigeration cassettes for light commercial applications. Such refrigeration systems are space constrained and, therefore, the heat exchangers (condenser and evaporator) must have a large area-to-volume ratio. In addition, such applications require a subfreezing evaporating temperature that induces the growth of a frost layer on the finned surface, which may block the evaporator if a proper defrost strategy is not used. Before completely blocking the evaporator, the frost layer depletes the heat exchanger performance by adding an extra thermal resistance and also by reducing the fan-supplied air flow rate. Understanding the way the frost forms on these compact heat exchangers and also the way the fan is affected by frost clogging is mandatory for the design of robust refrigeration systems and also to devise more efficient defrost strategies. In this study an experimental investigation on the frost accretion of tube-fin Evaporators considering the fan characteristics is carried out. To this end, a specially designed, constructed and calibrated closed-loop wind-tunnel facility was used. Experimental tests were carried out with four different (three wavy-fin and one louvered-fin) evaporator coils under different conditions. It was found that the frost formation rate increases with the air flow rate, supercooling degree and the density of fins. A strict relation between accumulated mass of frost, air-side pressure drop and cooling capacity was also observed. It was also noted that the fan characteristics play an important role on the evaporator thermal performance, indicating that under frosting conditions the fan-evaporator pair must be designed as a coupled system. Furthermore, for the same operating conditions, the louvered-fin evaporator showed to be more sensitive to the frost formation effects than the wavy-fin coils.
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a study of the air side heat transfer and pressure drop characteristics of tube fin no frost Evaporators
Applied Energy, 2009Co-Authors: Jader R Barbosa, Claudio Melo, Christian J L Hermes, Paulo J WaltrichAbstract:A study is presented on the influence of the air flow rate and surface geometry on the thermal-hydraulic performance of commercial tube-fin ‘no-frost’ Evaporators. A specially constructed wind-tunnel calorimeter was used in the experiments from which data on the overall thermal conductance, pressure drop, Colburn j-factor and Darcy friction factor, f, were extracted. Eight different evaporator samples with distinct geometric characteristics, such as number of tube rows, number of fins and fin pitch were tested. Semi-empirical correlations for j and f are proposed in terms of the air-side Reynolds number and the finning factor. A discussion is presented on the performance of the Evaporators with respect to specific criteria such as the pumping power as a function of heat transfer capacity and the volume of material in each evaporator.
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air side heat transfer and pressure drop in tube fin accelerated flow heat exchangers
2007Co-Authors: Paulo J Waltrich, Jader R Barbosa, Claudio Melo, Christian J L Hermes, Joel Boeng, Daniel S Vieira, Rodrigo KremerAbstract:An experimental analysis of heat transfer and pressure drop on the air-side of tube-fin Evaporators utilized in household 'frost-free' refrigerators is carried out in the present paper. The performance of an alternative evaporator concept, the so-called Accelerated Flow Evaporator (AFE) is systematically investigated for the first time. In this evaporator, the air-side cross sectional area decreases with the distance from the air flow inlet causing the air flow to accelerate and promote an enhancement of the air-side heat transfer coefficient. This heat transfer enhancement allows a reduction of the heat exchanger volume and hence its material cost. Comparisons between the experimental results and Computational Fluid Dynamics (CFD) simulations are also presented, with the purpose of validating a numerical methodology to be used as a design tool for this new type of evaporator.