Search

A three-dimensional numerical study on the combined effect of height as well as width tapering on the thermal performance of double taper microchannel is presented in this paper. The channel inlet width is considered as 300 µm, taper ratio on sidewalls and bottom wall are varied from 0 to 1 and 1 to 3.9, respectively. The thermal resistance ratio, average bottom wall temperature, temperature difference ratio, and pumping power ratio of the channel are evaluated for various flow rates, height, and width tapering. Results showed higher reduction of wall temperature with combined effect height as well as width tapering compared with straight channel. The optimal size of the micro channel to minimize the pumping power and average wall temperature on the constraint of heat flux and footprint area is found. The reduction in average bottom wall temperature is 17.34%, and pumping power ratio is 0.44 (56% power reduction) noted, respectively, at Reynolds number 340. Finally, optimal dimension of double taper microchannel is evaluated for better thermo-hydraulic performance.

Deep reinforcement learning (DRL) has been researched for computer room air conditioning unit control problems in data centers (DCs). However, two main issues limit the deployment of DRL in actual systems. First, a large amount of data is needed. Next, as a mission-critical system, safe control needs to be guaranteed, and temperatures in DCs should be kept within a certain operating range. To mitigate these issues, this article proposes a novel control method RP-SDRL. First, Residual Physics, built using the first law of thermodynamics, is integrated with the DRL algorithm and a Prediction Model. Subsequently, a Correction Model adapted from gradient descent is combined with the Prediction Model as Post-Posed Shielding to enforce safe actions. The RP-SDRL method was validated using simulation. Noise is added to the states of the model to further test its performance under state uncertainty. Experimental results show that the combination of Residual Physics and DRL can significantly improve the initial policy, sample efficiency, and robustness. Residual Physics can also improve the sample efficiency and the accuracy of the prediction model. While DRL alone cannot avoid constraint violations, RP-SDRL can detect unsafe actions and significantly reduce violations. Compared to the baseline controller, about 13% of electricity usage can be saved.

Single-atom catalysts (SACs) offer many advantages, such as atom economy and high chemoselectivity; however, their practical application in liquid-phase heterogeneous catalysis is hampered by the productivity bottleneck as well as catalyst leaching. Flow chemistry is a well-established method to increase the conversion rate of catalytic processes, however, SAC-catalysed flow chemistry in packed-bed type flow reactor is disadvantaged by low turnover number and poor stability. In this study, we demonstrate the use of fuel cell-type flow stacks enabled exceptionally high quantitative conversion in single atom-catalyzed reactions, as exemplified by the use of Pt SAC-on-MoS2/graphite felt catalysts incorporated in flow cell. A turnover frequency of approximately 8000 h−1 that corresponds to an aniline productivity of 5.8 g h−1 is achieved with a bench-top flow module (nominal reservoir volume of 1 cm3), with a Pt1-MoS2 catalyst loading of 1.5 g (3.2 mg of Pt). X-ray absorption fine structure spectroscopy combined with density functional theory calculations provide insights into stability and reactivity of single atom Pt supported in a pyramidal fashion on MoS2. Our study highlights the quantitative conversion bottleneck in SAC-mediated fine chemicals production can be overcome using flow chemistry.

To capitalize the advantage of oblique fin heat sink (OFHS) with Al2O3–water nanofluids of different volumetric concentration (1, 2, and 4%), a comprehensive computational analysis has been...

High efficiency condenser coil providing higher heat removal capacity with lower air-side pressure drop is desirable. However, coil designs with different fin patterns for higher heat transfer incur large air-side pressure drop and thus consume more energy. In this study, the thermal and hydraulic performances of the condenser coil design with plain fin, corrugated fin, and novel oblique-shaped tube are evaluated numerically with a 3-fin and 3-tube model in comparison with a conventional circular tube coil. This novel oblique-shaped tube design features smaller air recirculation zone behind the tube so as to increase effective heat transfer area and to lower air-side pressure drop. The numerical results show that, for the plain fin configuration, the shape of the oblique tube is important in reducing the recirculation zone on the fin, thus increasing effective heat transfer area and heat transfer amount up to 14% and reducing the air-side pressure drop up to 14% at the same inlet air velocity as compared with the circular tube coil. With corrugated fin guiding and promoting air speed, causing more flow separation zones, it is detrimental to the thermal-hydraulic performance of the oblique tube coil. The discussion of choice of materials for tube and fin, such as copper, brass, and aluminium, is provided with possible manufacturing processes required for realizing potential high performance coil design. Parametric optimisation on the best performing plain fin oblique tube design shows that, for the design of condenser coil with oblique tube, there is no optimal FPI, but it is recommended to keep the ratio of tube pitch to tube frontal length/diameter to about 3 to 3.5.

A stable flow boiling operation is key to enhancing the two-phase cooling performance of microchannel heat sinks. To this end, a novel heat sink is developed which integrates a 300 μm × 600 μm straight microchannel array in the upstream region with a 25 mm × 600 μm microgap channel in the downstream region of a 25 mm × 25 mm copper heat sink. Flow boiling experiments are conducted using de-ionized water at 5 different mass fluxes in the range of 100–399 kg/m2 s, supplied at a fixed inlet temperature of 85.5 °C. The downstream heat transfer coefficient in the microgap section shows an M-shaped profile with increasing heat flux and vapor quality. Stable boiling conditions are prevalent across a large span of operating heat flux. Instabilities are experienced only for a short range of low heat flux following ONB. The stabilizing effect is attributed to the larger flow cross-section area offered by the downstream microgap section that allows expanding vapor bubbles to evacuate with lesser hindrance. Flow visualization reveals that a stable annular flow regime is established at moderate to high heat flux during which the stable boiling operation is observed. Thin film evaporation taking place during annular flow conditions results in an increasing trend in the downstream heat transfer coefficient from moderate to high heat flux. Pressure drop associated with the hybrid heat sink is found to be modest and reaches a maximum of 6 kPa at the highest mass flux of 399 kg/m2 s. A brief comparison of this heat sink is made with its straight microchannel and microgap heat sink counterparts. The hybrid heat sink shows a heat transfer performance that is superior to the microgap heat sink while poorer than the straight microchannel heat sink although it offers a better boiling stability than the straight microchannel heat sink. It however lowers the pressure drop compared to the straight microchannel heat sink.

This paper presents new development of aluminium oblique-tube condenser coils with plain fins manufactured by controlled atmospheric brazing (CAB) for air-conditioning applications. The CAB technique provides good quality joints and thermal contact between oblique tubes and fins for more effective heat transfer. Performances of seven oblique-tube coils with different numbers of tubes per pass (i.e. 1, 2, 3, 4, 5, 7 and 14) were assessed against those of a commercial circular-tube coil with corrugated fins. The contribution of the slender oblique tubes with plain fins reduced air pressure drop penalties by 20.9–30.9% for 1–3 m/s air velocity. Thermal performance of the 1-tube-per-pass coil surpassed that of the commercial coil albeit not energy-saving. On the other hand, the 4-tube pass coil when matching with the heat transfer performance of the 3- and 2-tube-per-pass coils and the circular-tube coil saved water-side pump energy as high as 27.9%, 36.3% and 38.7%, respectively.

Numerical and experimental study of minichannel heat sinks (MCHS), with air as a cooling medium, has been performed for straight channel heatsink (SC HS), wavy channel heatsink (WC HS) and branch wavy channel heatsink (BWC HS) to investigate thermal and hydraulic performances. After considering both the conduction and the convection as a mode of heat transfer, numerical computation of three-dimensional conjugated heat transfer between the heat sink and air flow has been performed by using full domain simulation in ANSYS Fluent. Laminar model is selected for the study of air flow and heat transfer as the Reynolds number varied from 300 to 1900 for a different range of air flow rates. Heating powers of 20 watt and 30 watt are applied at the base of the heat sink. Followed by detailed experimentation, the numerical results are also examined to get acquainted with the flow fields and their roles in the thermal performance of individual heat sinks. It has been found that the thermal-hydraulic performance factor of BWC HS exhibited superiority over SC HS and WC HS. It is also noticed that significant improvements in the junction temperature of BWC HS have been achieved. Experimental results have been validated with numerical results.

In this paper, a new combined cooling and power (CCP) system has been proposed which enables harnessing of very low-grade heat (< 100 °C) in an effective way. The novel system configuration uses two vapour generators to extract more heat from heating stream and employs ejector in a unique way to serve two purposes; (i) to reduce pressure at the expander outlet for more power output (ii) vapour compression for refrigeration. The system is modelled in EBSILON which is a commercial thermal design and optimization tool. For ejector performance prediction, a new on-design optimal performance analytical model is used which is developed in EES (Engineering Equation Solver). The parametric study shows that when the evaporator pressure is changed from 0.8 bar (9.4 °C) to 1.2 bar (19.5 °C) the output of the system is increased by 58 % and when the condenser pressure is changed from 1.8 bar (30.5 °C) to 2.2 bar (36.2 °C) the system output is dropped by 56.4 %.

As the air-side heat transfer is controlling the efficiency of fin and tube heat exchangers (FTHX), the thermal enhancement of FTHX relies more on the air-side. A theoretical model of the baseline FTHX is built using ANSYS Fluent which is then validated by wind tunnel experiments. After analysing the simulation results of the baseline FTHX, two novel air-side fin configurations are proposed. The first design can guide more airflow to the back of the tubes to mitigate wake zones. For the second design, topology optimisation is used to significantly increase the heat transfer area at the air-side with minimised pressure drop penalty. To further improve the two designs, parametric studies are conducted through which optimal design parameters are obtained. Comparing with the baseline FTHX, the optimal guiding channel fin design and topology optimisation fin design can dissipate 8.5% and 7.0% more heat respectively, or consume 41.4% and 33.3% less fan power respectively. As such, the proposed enhanced air-side fin designs are promising candidates for improving the efficiency of FTHXs.

This work presents a method of designing an air heat sink with forced convection by topology optimization. Both pressure drop and heat transfer performances are evaluated. To reduce computational cost, a 2D two-layer model is first developed and implemented in COMSOL Multiphysics to represent three-dimension fully conjugate heat transfer modeling. It has been shown to be accurate in temperature field prediction and able to capture trends of pressure drop variation. Through a multi-stage optimization process, a non-conventional fin structure is created. The optimized structure is then manufactured and experimentally validated. Compared to a conventional straight channel heat sink, the topology optimized heat sink can achieve lower junction temperatures with the same pumping power or requires lower pumping powers for maintaining the same junction temperature. Furthermore, full 3D numerical analysis by ANSYS Fluent is performed to study the detailed characteristics of the topology optimized heat sink. It shows that the non-conventional layout of the fins introduces strong mixing effect, continuous boundary layer interruption and local high speeds, which all contribute to heat transfer enhancement.

A solid-desiccant system that utilizes low grade heat is potentially a viable add-on to conventional HVAC systems since it can help reduce power consumption significantly, for achieving indoor thermal comfort conditions. In contrast to desiccant wheels which carry out adiabatic dehumidification, isothermal dehumidification process that may be realized by a cross-flow heat exchanger is much more efficient. In this paper, a novel mathematical model is developed to simulate heat and mass exchange phenomena of a desiccant coated fin tube heat exchanger (DCFTHX). This model takes solid side mass transfer resistance as well as fin efficiency into consideration. The model is validated using experimental results in the literature. It is also compared against simplified models to establish its utility. A parametric study is then conducted to investigate the effects of geometrical parameters as well as mass flow rate of water and air velocity on dehumidification and adsorption heat removal performance of the DCFTHX as well as the performance of the augmented air-conditioning system under warm and humid ambient conditions. Under the range of parameters and conditions simulated, if low grade waste heat (50 °C hot water) is available for regeneration, integration of DCFTHX sub-system with a conventional air conditioning system can yield as high as 31% energy savings (even when the additional power consumed by pumps and blower fans is accounted for).

Poor airflow management attributed to hot air recirculation and cold air by-pass adds massive thermal inefficiencies to air-cooled data center’s cooling system. The containment system usually was adopted to mitigate such drawbacks by isolating the space using a physical partition wall, such as hot aisle containment. Although the recirculation and by-pass can be effectively suppressed, the consequent high backpressure built up in the containment reducing the server air flowrate and possibly increasing the server temperature beyond the acceptable limit. This study presents a new enclosure mechanism for air-cooled data center using a vortex flow as an alternative to the conventional hot aisle containment to reduce the hot recirculation and, at the same time to retain the server air flowrate. The experiments conducted in a half-scale test cell (2-times smaller than a typical data center hall in length scale) examined vortex containment's effectiveness. The experiment results revealed that the average Supply Heat Index (SHI) in vortex hot aisle containment could be reduced to 0.11. Although vortex hot aisle containment did not enclose all hot recirculation compared to the full containment deployed in the benchmark layout, the relieved backpressure (the high pressure in the hot aisle) retains the rack flowrate by 42% (2.1 m/s in the vortex hot aisle containment with a diameter 1.2 m (VFL1200) layout compared to 1.2 m/s in full containment) and results in a 4.3% lower normalized server temperature (1.03 in VFL1200 layout compared to 1.076 in full containment CL). In the end, the validated CFD model provides more insights on vortex containment characteristics, such as the vortex strength and pressure distribution.

Airflow management in the data center is one of the most researched topics to achieve uniform airflow for effective thermal management. Recently, the Fan Wall Cooling (FWC) system is popular in air-cooled data centers due to the advantage of no raised floor plenum. In this research, a validated CFD model is used to study the FWC performance with different configurations to gain insights for the optimum design. Five new configurations are proposed to optimize the FWC performance along with ordinary FWC. Each of them has a different rack-tilted and row-tilted angle. The results show that tilting the rack by 15° towards the supply air direction can increase the rack air flowrate by 2.5% (800 CFM), and reduce the maximum normalized server temperature ( T s e r , m a x ∗ ) by 7% from 1.30 to 1.21. The CFD simulations also reveal that all new configurations can effectively mitigate the airflow maldistribution. Lastly, the FWC exhibits superior thermal performance compared to the conventional Underfloor Air Delivery (UFAD) and the Overhead Air Supply (OHAS) methods.

The thermal and hydraulic performances of an air-cooled, planar, oblique-finned heat sink (OF HS) were investigated for two oblique angles: 30° and 45°. The conjugate heat transfer between the heat sink and the air flow were computed numerically in ANSYS Fluent for a range of air flow rates for the smallest periodically repeating portion of the heat sink. The RNG k-e turbulence model with enhanced wall treatment was used to solve the fluid flow and heat transfer. Followed by the experimental validation, the numerical results were scrutinized further to understand the effects of the flow field on the measured heat transfer performances. Apart from boundary layer disruption, vortices generated within the secondary channels due to flow separation particularly improved the advection component of the convective heat transfer, resulting in a heat transfer enhancement, exceeding the pressure drop penalty. The strong flow mixing, showing chaotic behavior, enabled a more uniform increase in the air temperature in the streamwise direction, utilizing the cooling potential of the air flow more effectively. 30° oblique-finned heat sink was observed to induce higher rates of secondary flow rates and improve the heat transfer performance more than its 45° counterpart. Due to the migration of the flow in the direction of the secondary channels, the heat transfer enhancement was compromised at high Reynolds numbers, the pressure drop penalty exceeded the heat transfer enhancement, causing a reduction in the thermal-hydraulic performance factor. The effects of flow migration on the flow and temperature fields were investigated with full domain numerical simulations. Experimental investigations showed significant improvements in the junction temperatures.

Rapid advancement toward miniaturization has emerged with confront for superior heat dissipation techniques. Of all the available cooling systems, microchannel-based cooling systems stand out to provide better cooling performance through superior heat removal abilities. In the present study, the cooling performance and hydraulic flow characteristics of a radial curved microchannel with three curvature ratios were numerically investigated and compared with a radial straight microchannel. Unlike the conventional straight microchannels, curved channels possess better fluid mixing as a result of the centrifugal force caused due to curvature. This phenomenon has a significant effect on heat transfer and fluid flow characteristics. Work on radial curved microchannels has been scarce and there is a lot of potential to augment the heat transfer with lower pumping power particularly with a central inlet. A three-dimensional conjugate heat transfer analysis was carried out for three radial curved microchannels and a radial straight microchannel using the ANSYS Fluent commercial software with the Reynolds number range of 125–275. The results showed a Nusselt number increment of 36.38% for radial curved microchannels when compared to the radial straight microchannel. Further, the lowest average wall temperature was noted for the radial curved microchannel with a curvature ratio of 0.17 which was 15.63 °C lower when compared to that in a radial straight microchannel for the same Reynolds number. Contours of velocity and temperature are presented at various locations along the stream to aid the results. The overall performance of all three radial curved microchannels was found to be higher than that of the radial straight microchannel in the Reynolds number range considered, out of which the maximum performance factor of 1.245 was obtained for the radial curved microchannel with a curvature ratio of 0.17 as compared to the radial straight microchannel.

In the 21st conference of the parties (COP21) of United Nations, both renewable energy and energy efficiency were recognized as essential means for low-carbon future. It is estimated that around 20-50 % of energy input to industries is lost as waste heat. Low-grade heat is abundantly available from renewable sources like solar, biomass and geothermal. There is a need to develop technologies which can utilize very low-grade heat (

The practical application of microfluidic liver models for in vitro drug testing is partly hampered by their reliance on human primary hepatocytes, which are limited in number and have batch-to-batch variation. Human stem cell-derived hepatocytes offer an attractive alternative cell source, although their 3D differentiation and maturation in a microfluidic platform have not yet been demonstrated. We develop a pump-free microfluidic 3D perfusion platform to achieve long-term and efficient differentiation of human liver progenitor cells into hepatocyte-like cells (HLCs). The device contains a micropillar array to immobilize cells three-dimensionally in a central cell culture compartment flanked by two side perfusion channels. Constant pump-free medium perfusion is accomplished by controlling the differential heights of horizontally orientated inlet and outlet media reservoirs. Computational fluid dynamic simulation is used to estimate the hydrostatic pressure heads required to achieve different perfusion flow rates, which are experimentally validated by micro-particle image velocimetry, as well as viability and functional assessments in a primary rat hepatocyte model. We perform on-chip differentiation of HepaRG, a human bipotent progenitor cell, and discover that 3D microperfusion greatly enhances the hepatocyte differentiation efficiency over static 2D and 3D cultures. However, HepaRG progenitor cells are highly sensitive to the time-point at which microperfusion is applied. Isolated HepaRG cells that are primed as static 3D spheroids before being subjected to microperfusion yield a significantly higher proportion of HLCs (92%) than direct microperfusion of isolated HepaRG cells (62%). This platform potentially offers a simple and efficient means to develop highly functional microfluidic liver models incorporating human stem cell-derived HLCs. Biotechnol. Bioeng. 2017;114: 2360-2370. © 2017 Wiley Periodicals, Inc.

In this study, the conjugate heat transfer performance of an enhanced planar heat sink design, comprising cross-connected alternating converging–diverging channels, was analyzed for forced air convection conditions. Numerical simulations were performed in ANSYS Fluent 15.0 using the RNG k–e turbulence model accompanied by the enhanced wall treatment option to resolve the air flow and evaluate the heat transfer. Numerical results, which were validated experimentally, were utilized to investigate the flow and the temperature fields. The converging–diverging channel sections induced secondary flows through the cross connections, repeatedly disturbing the thermal and hydraulic boundary layers over the leading edges of the fin sections. The performance of the proposed heat sink design was benchmarked against the conventional straight channel heat sink of equivalent dimensions. Significant heat transfer enhancement was observed. However, the vortices, generated as a result of the separation of the secondary flows, were observed to prevent the heat transfer performance from being further improved and cause an excessive increase in the pressure drop penalty.

Raised-floor data center supplying the cold air to servers through the perforated tiles encounters the thermal inefficiency from the cold air by-pass and the hot air recirculation. Although the containment solutions solve the problem, studies [1]–[4] have emphasized the potential rise of the energy cost if extensive leakages are presented or poorly maintained. In this study, an experiment was conducted to evaluate a new approach to minimize the recirculation and by-pass other than the conventional containment system, i.e. Vortex-Enhanced Hot Aisle Containment (VEHAC) using a scale model with scale ratio 11.25. The formation of a vortex observed in the experiment built a “virtual wall” at special oriented hot aisle preventing the hot air recirculating back to cold aisle. Server inlet air temperature was improved by 0.4 °C in the scale model measured during the experiment.

Data center thermal management is one of the focused research areas worldwide to optimize system performance to reduce electric power consumption [1]. One of the ways is effective airflow management by aisle containment. To achieve this, in this study we proposed vortex hot aisle containment first time in data center research community. The rack arrangement is designed in such a way that the hot aisle airflow can create the vortex due to pressure gradients. To characterize this airflow, we performed initial CFD simulations [2] and started the experimentation. This paper focuses on the experimental details of this study. Four 32U racks with 6U server simulators were tested in raised floor plenum data center facility at School of Commuting in the National University of Singapore. Initial steady-state experiments were performed for vortex hot aisle containment to monitor the rack inlet and outlet air temperature for varying heat load and server fan speed. Results show the favorable pressure variations to generate the vortex airflow inside the vortex hot ails containment and expect to give more insights with some more experiments.

This study investigates the thermal performance of laminar single-phase flow in an additively manufactured minichannel heat exchanger both experimentally and numerically. Distilled water was employed as the working fluid, and the minichannel heat exchanger was made from aluminum alloy (AlSi10Mg) through direct metal laser sintering (DMLS). The minichannel was designed with a hydraulic diameter of 2.86 mm. The Reynolds number ranged from 175 to 1360, and the heat exchanger was tested under two different heat fluxes of 1.5 kWm−2 and 3 kWm−2. A detailed experiment was conducted to obtain the thermal properties of AlSi10Mg. Furthermore, the heat transfer characteristics of the minichannel heat exchanger was analyzed numerically by solving a three-dimensional conjugate heat transfer using the COMSOL Multiphysics® to verify the experimental results. The experimental results were also compared to widely accepted correlations in literature. It is found that 95% and 79% of the experimental data are within ±10% range of both the simulation results and the values from the existing correlations, respectively. Hence, the good agreement found between the experimental and simulation results highlights the possibility of the DMLS technique as a promising method for manufacturing future multiport minichannel heat exchangers.

Cold air by-pass and hot air recirculation are two primary sources of inefficiencies in data center thermal management. Although some studies have been conducted to investigate the cause of such issues [1] [2], few attempts were reported to improve the situation by changing the rack layout configuration. A new rack layout, called Vortex Flow Layout (VFL) configuration is proposed in this study. The configuration is simulated under fully, partial and no containment cases to evaluate its performance. Studied from a steady state CFD model utilizing the vortex flow effect, the hot air recirculation is found to be suppressed by 3 °C and 5.3 °C in partial containment and open configuration respectively. It is found that the formation of a vortex traps the hot server-outlet air in the vortex core that helps to restrict the recirculation. The VFL partial containment shows the best performance with high recirculation suppression effect and reduced fan power.

Advanced cooling solutions are crucial for the further development of modern technologies. Additive manufacturing (AM) helps to remove many of the conventional design constraints and thereby enables the realization of novel heat sink topologies and geometries that lead to enhanced thermal performance. In this study, porous fin arrays with minimum cavity and channel size of 0.3mm are designed and manufactured using Selective Laser Melting (SLM) technology. AlSi12 alloy powders are adopted to print directly on the copper base. Stable connection is achieved between two materials and thermal resistance is reduced to minimum. The heat transfer performance of heat sinks with different porous layers are evaluated through pool boiling experiments with deionized water. The benchmark test involves a plain copper surface with a footprint of $10\times 10\mathrm{mm}$ , same as the base used for additive manufacturing. High speed visualization results are combined with the measured boiling data to assess the two-phase heat transfer performance of heat sinks. Superior heat removal performance is achieved with SLM manufactured porous fin arrays with the highest heat transfer coefficient (HTC) enhancement of 80% and critical heat flux (CHF) enhancement of more than 170%. The performance boost is mainly attributed to the larger number of nucleation sites, increased heat transfer area, capillary-assisted suction, and separated liquid-vapor pathways associated with the porous fin array designs. The present study showcases the combination of complex structure design with SLM technology and would provide useful insight for the development of future high performance heat sink designs and the optimization of AM process.

Internally cooled and heated desiccant-coated heat and mass exchangers (ICHDHMX) driven by low-grade heat are very attractive owing to their energy-saving potential, especially for applications where substantial moisture removal (such as air-conditioning) is a necessity. In this paper, we derive equations for the performance of an ideal ICHDHMX, allowing us to define humidity-ratio effectiveness (εY) and relative-humidity effectiveness (εRH) such that their values approach 1 as the performance approaches that of an ideal ICHDHMX. Besides an equation-based approach, an easy-to-use psychrometric-chart based approach is presented to determine the performance of an ideal ICHDHMX. We invoke conservation principles to ascertain whether or not it is feasible to use the ICHDHMX for a given set of inlet conditions of air and water streams for dehumidification and regeneration. The dimensions of the ICHDHMX can be determined using this methodology, not even requiring knowledge of a tuning parameter unless a precise outlet specific humidity is required. Simulations are conducted for cases involving three incoming hot water temperatures (38, 44 and 50 °C) and several mixing ratios of room return air (25 °C at 0.011 kg/kg dry air) and outdoor air (32 °C at 0.02 kg/kg dry air), typical of warm and humid weather conditions. For all cases, the cool-water inlet is fixed at 30 °C. The results show that even when the dehumidification air-stream humidity is high, if the regeneration air-stream humidity is low (typical of room-exhaust air), the operation of an ICHDHMX is feasible using a low regeneration temperature of only 38 °C. When the regeneration temperature is 50 °C, the exchanger can operate under the complete range of humidity conditions tested. A cooling coefficient of performance up to 9.8 and effectiveness value up to 0.88 is realized, while the fluid power required is generally very low. These findings substantiate the case for commercial adoption of this technology for air-conditioning.

Flow boiling in microchannels has been receiving a lot of attention in recent years because of its high heat flux removal capabilities at low flow rates and low pumping power. An important aspect of flow-boiling experiments is prediction or detection of the prevalent flow-regime. Currently, most researchers use high-speed camera for flow visualization for regime detection. However, in some cases due to limitations of the experimental setup and test-piece, such as multi-layer cooling of 3D IC stack, this may not be feasible. In this paper, the influence of flow-regime on frequency domain of local temperature data of the wetted surface is studied. Experiments have been performed synchronously with high speed flow visualization on a single microchannel with width and length of 2.54 mm and 25.4 mm respectively. The microchannel heights tested were 0.14 , 0.28 and 0.42 mm . De-gassed, de-ionized water was used as the working fluid. Mass fluxes tested ranged from 200 to 1000 kg / ( m 2 s ) . Depending on the prevalent flow regime, some of the highest of peak amplitudes in the frequency domain were quite distinct. Within the bounds of current experimental parameters, it is concluded that local transient temperature data can be a potential diagnostic tool for detection of flow-regimes. (A shorter version of this paper was presented at the 7th International Conference on Applied Energy (ICAE2015), March 28–31, 2015, Abu Dhabi, UAE (Original paper title: “Temperature transients for detection of flow-regimes in a mini/microchannel” and Paper No.: 430).)

This paper presents a simplified equivalent circuit model, which exploits the electric–hydraulic analogy and electric–thermal analogy, to predict the mass flow distribution and temperature distribution in an oblique fin array used in enhanced heat transfer applications. Methods to obtain accurate correlations for calculation of flow-dependent hydraulic ‘resistances’ are outlined and developed for both primary and secondary channels in the oblique fin array. Appropriate Nusselt number correlations and thermal resistance models are also employed to predict the temperature distribution associated with the mass flow distribution. Detailed full-domain numerical (CFD) simulations are performed to obtain parameters for the hydraulic resistance correlations, and also to serve as benchmarks for the proposed equivalent circuit model. Detailed comparisons between the results of simplified model and numerical simulation showed that the simplified model can accurately predict the mass flow distribution and temperature distribution, within ±5%, for varying fin number, aspect ratio, fin pitch, fin length, oblique angle and inlet velocity. Slightly higher deviations of mass flow prediction are observed for high inlet velocities as a result of the presence of vortices close to the trailing edge of the oblique fin region.

The fluid flow and heat transfer behaviors in the novel sinusoidal micro-channels with secondary branches were examined both numerically and experimentally. Secondary branches, inspired by the oblique fin design were added in an alternating fashion (±45°) to the peaks and the troughs of the wavy micro-channel configuration to enable cross-channel mixing. Two designs with different amplitude of waviness were benchmarked against the optimal wavy channel configuration from the Reynolds number range of 50–200. An increase in heat transfer performance (when compared to the reference conventional wavy design) was observed when secondary branches were added, albeit with a pressure drop penalty. However, when another design with lower amplitude of waviness was introduced and compared to the reference geometry, superior heat transfer performance was observed without pressure drop tradeoff. The velocity and temperature profiles from the simulations were obtained to explain the hydrodynamic and heat transfer behaviors. The combination of Dean’s vortices induced by the wavy channel design and the cross-channel mixing enabled by the secondary branches showed potential to further enhance convective heat transfer (at reasonable pressure drop penalties) due to the increased intensity of span-wise fluid mixing and also boundary layer disruptions. The attractiveness of the introduction of secondary branches lies in the potential to further enhance the heat transfer performance of the already established wavy channel configuration with minimal pressure drop penalty.

The oblique-finned structure has been proven to exhibit significant enhancements in heat transfer in both single-phase and two-phase modes of operation coupled with slight increment in pressure drop. These enhancements come with the mitigation of two-phase flow instability compared to the conventional straight-finned microchannels, which encourages further parametric studies on the fin geometry. In the present work, flow boiling experiments are conducted in oblique-finned microchannels using the FC-72 dielectric fluid with varying secondary channel widths to investigate its effects on two-phase heat transfer, pressure drop and flow boiling instabilities. Three different secondary channel widths of 0.15 mm, 0.30 mm and 0.45 mm are tested with mass fluxes ranging from 197 kg/m 2 s to 394 kg/m 2 s, and effective heat fluxes from 14.9 W/cm 2 to 70.2 W/cm 2 with the aid of high-speed visualisations. From the experiments and flow visualisations, it is found that increasing the width of the secondary channels has adverse effect on heat transfer performance and critical heat flux limit due to the inability of the microchannels to suppress flow boiling instabilities. Pressure drop penalty of the larger secondary channel configurations are higher as a result of greater amount of flow diversion, which amplifies flow migration from the draining edge to the filling edge of the oblique-finned array.

Flow boiling experiments are conducted in oblique-finned microchannels with different oblique angles to investigate its effects on two-phase heat transfer, pressure drop and instabilities. These studies are carried out with FC-72 dielectric fluid at three different oblique angles of 10°, 30° and 50°. Three mass fluxes ranging from 194 kg/m 2 s to 386 kg/m 2 s, and effective heat fluxes from 14.9 W/cm 2 to 70.0 W/cm 2 are tested with high-speed flow visualisations to explore the physical boiling phenomenon in the microchannels. Results show that heat transfer performance increases with oblique angles from 10° to 50° due to the increase in secondary channels which promote flow mixing. This causes an increase in pressure drop from 10° to 30° oblique angles due to the increase in flow separation and flow diversion. Pressure drop, however, is almost negligible when the oblique angles are increased from 30° to 50°. As a result, the 50° oblique-finned microchannels presents an attractive option for further parametric studies.

The challenges that microchannel flow boiling technology faces are the lack of understanding of underlying mechanisms of heat transfer during various flow boiling regimes and a dearth of analytical models that can predict heat transfer. This paper aims to understand flow boiling heat transfer mechanisms by analyzing results obtained by synchronously captured high-speed flow visualizations with local, transient temperature data. Using Inverse Heat Conduction Problem (IHCP) solution methodology, the transient wetted surface heat flux and temperature as well as heat transfer coefficient are calculated. These are then correlated with the visual data. Experiments are performed on a single microchannel embedded with fast response temperature sensors located (630 µm) below the wetted surface. The height, width and length of the microchannel are 0.42 mm, 2.54 mm and 25.4 mm respectively. De-ionized, de-gassed water is used as the working fluid. Two heat fluxes are tested at each of the mass fluxes of 182 kg/(m2s) and 380 kg/(m2s). Because of vapor confinement, slug flow is observed for the tested conditions. The present study provides detailed insights into the effect of various events such as passage of vapor slug, 3-phase contact line, partial-dry-out and liquid slug on transient heat transfer coefficient. Transient heat transfer coefficient peaks when thin film evaporation mechanism is prevalent. The peak value is influenced by the distance of bubble incipience as well as downstream events obstructing the flow. Heat transfer coefficient during the passage of liquid slug and 3-phase contact line were relatively lower for the tested experimental conditions.

Owing to the transient nature of microchannel flow-boiling, gaining an in-depth understanding of the associated trends in time-averaged heat transfer variables as well as mechanisms of heat transfer is quite challenging. This work studies sub-cooled flow-boiling of de-ionized water in a microchannel. The micro-channel heights studied are 0.14, 0.28 and 0.42 mm while the tested mass fluxes range from 200 to 1000 kg/(m2s). Slug flow regime is observed under most of the tested conditions. Transient wetted surface heat flux and temperature as well as heat transfer coefficient are evaluated from the wall temperature data captured at a high data-logging frequency (synchronously with flow-visualizations). Heat transfer mechanisms as well as the trends observed in plots associated with time-averaged heat transfer variables are explained from the perspective of transient data correlated to the observed flow-boiling phenomena. Thin-film evaporation emerges as the prominent heat transfer mechanism. Two-way coupling between transient heat transfer variables and the boiling phenomena is evident.

Flow maldistribution compromises thermal and hydraulic performances of heat sinks. The oblique-finned heat sink is a multichannel heat sink comprising parallel (primary) and collinear oblique (secondary) channels and it was reported to suffer from a self-induced flow migration in liquid and air cooling. In this study, a novel tapered inlet/outlet flow manifold was proposed for the planar oblique-finned heat sink. Two heat sink configurations (oblique angles of 30° and 45°) and three flow manifold configurations (of 5%, 10%, and 20% wider flow domains) were investigated. Flow migration characteristics were studied via numerical simulations. Wind tunnel measurements, supported by infrared thermography, were performed to characterize thermal and hydraulic performances on a wide range of air flow rates. The flow manifold was observed to provide a favorable distribution of secondary flows in the spanwise direction of the heat sink. Up to 50% increase in Nusselt number with a negligible increase in pressure drop was observed with the 45° oblique-finned heat sink once the flow manifold was incorporated. 30° oblique angle −10% flow manifold combination provided the lowest heat sink temperature and the most uniform wall temperature. The novel tapered inlet/outlet flow manifold is the first effort reported in the literature to reduce the adverse effects of flow migration through the planar oblique-finned heat sinks.

This paper presents the modeling and optimization of an ejector refrigeration system (ERS). Engineering Equation Solver (EES) has been used for modeling. These models have been validated against published experimental results. Refrigerant R245fa is used as a working fluid. A new analytical model for predicting the optimum (on-design) performance (entrainment ratio) of ejectors is presented. The analytical model of the ejector is validated against two sets of published experimental data and good agreement is established with an average error of 3.4%. The presented ERS model has also been validated with two sets of published experimental results and an average error of 2.9% is obtained. The ERS model is further compared with a model developed in EBSILON (a commercial tool). The results show that for the optimum design of an ERS, the generator pressure increases linearly with heat source temperature while the output (refrigeration effect) increases exponentially. For the case study, the designed ERS produces 1.8 MW of cooling with estimated annual savings of S$0.42 million while operating at 0.3 COP. The presented models can be used for designing ejector and ERS for various applications.

This paper presents a new fin pattern for liquid-cooled microchannel heat sinks. It is derived from topology optimized heat sink structures reported in our previous paper. The new fin pattern is composed of two types of fins, oblique fin and trapezoidal fin, and they are arranged in a semi-wavy manner. Numerical simulation is implemented to examine its heat transfer and fluid flow characteristics in detail. A flow phenomenon that is similar to Dean’s vortices is observed and results in enhanced flow mixing. The non-continuous fin pattern also avoids growth of thick boundary layers as they are initiated at the leading edges of each fin. Furthermore, size optimization based on the response surface method is performed to balance the heat transfer enhancement and pressure drop penalty. The new fin pattern is then compared with five other commonly used fin designs and shows the best performance in the studied flow range.