Xin Cui

Bio: Xin Cui is an academic researcher from Xi'an Jiaotong University. The author has contributed to research in topics: Evaporative cooler & Air conditioning. The author has an hindex of 13, co-authored 47 publications receiving 626 citations. Previous affiliations of Xin Cui include National University of Singapore.

Thermal response of annuli filled with metal foam for thermal energy storage: An experimental study

Abstract: Latent heat thermal energy storage is a practical way to solve the intermittent of solar energy. However, the inherent low thermal conductivity of phase change materials (PCMs) hampers their widely-used applications. In this study, a visual experiment test rig was designed to investigate the effect of open-cell metal foam embedded into paraffin (PCM) on the thermal response of a shell-and-tube unit during charging process. High temperature water, selected as heat transfer fluid (HTF), was injected from top of copper tube. The evolution of solid-liquid interface in inside and outside views was captured and recorded by a high-definition camera. 3D interface model was reconstructed based on these interface images and melting fraction was thus calculated. T type thermocouples were arranged separately on the radial and axial positions inside the PCM. Heat transfer annuli filled with PCM and composite PCM were tested under different HTF injection velocities. Experimental results demonstrated that the involvement of open-cell metal foam can dramatically enhance the efficiency of thermal energy storage. Compared with pure PCM, the full melting time of composite PCM was reduced by 64%, and the temperature distribution was more uniformity. Increasing the injection velocities of HTF made little contribution to promoting charging process of two samples.

Performance evaluation of an indirect pre-cooling evaporative heat exchanger operating in hot and humid climate

Abstract: A hybrid system, that combines an indirect evaporative heat exchanger (IEHX) and a vapor compression system, is introduced for humid tropical climate application. The chief purpose of the IEHX is to pre-cool the incoming air for vapor compression system. In the IEHX unit, the outdoor humid air in the product channel may potentially condense when heat is exchanged with the room exhaust air. A computational model has been developed to theoretically investigate the performance of an IEHX with condensation from the product air by employing the room exhaust air as the working air. We validated the model by comparing its temperature distribution and predicted heat flux against experimental data acquired from literature sources. The numerical model showed good agreement with the experimental findings with maximum average discrepancy of 9.7%. The validated model was employed to investigate the performance of two types of IEHX in terms of the air treatment process, temperature and humidity distribution, cooling effectiveness, cooling capacity, and energy consumption. Simulation results have indicated that the IEHX unit is able to fulfill 47% of the cooling load for the outdoor humid air while incurring a small amount of fan power. Consequently, the hybrid system is able to realize significant energy savings.

Numerical simulation of a novel energy-efficient dew-point evaporative air cooler

Abstract: We present simulation results on a novel dew-point evaporative air conditioner which was designed based on a counter-flow closed-loop configuration consisting of separated working channels and product channels. The novel evaporative air conditioner is able to cool air to temperature below ambient wet-bulb temperature and approaching dew-point temperature. To investigate the performance of the evaporative air cooler under a variety of conditions, the Eulerian–Lagrangian computational fluid dynamics (CFD) model was adopted. We validated the model by comparing the temperature distributions and outlet air conditions against experimental data. The numerical model showed good agreement with the experimental findings to within ±10%. Impacts due to the inlet air condition, the air flow velocity, the dimension of the airflow passages, and the product-to-working air flow ratio on the cooler performance were analyzed. Simulation results have indicated that the novel dew-point evaporative air conditioner is able to achieve a higher wet-bulb and dew-point effectiveness with lower air velocity, smaller channel height, larger length-to-height ratio, and lower product-to-working air flow ratio.

Fundamental formulation of a modified LMTD method to study indirect evaporative heat exchangers

Abstract: An analytical model for indirect evaporative heat exchangers has been developed via a modified log mean temperature difference (LMTD) method designed for sensible heat exchangers. The original LMTD method is judiciously modified to extend its computing method to indirect evaporative cooling systems where latent heat transfer is involved. The analytical model is validated by comparing its prediction of thermal performance with experimental data of a counter-flow dew-point evaporative cooler and a cross-flow indirect evaporative cooler. Predictability of the modified LMTD method has a maximum discrepancy of ±8% when compared to experimental data. The model has been demonstrated to be a practical method to provide an accurate result with a short computational time. Several case studies are structured to illustrate that the modified LMTD method is suitable for designing and analyzing indirect evaporative heat exchangers.

Experimental study on the solidification process of fluid saturated in fin-foam composites for cold storage

Abstract: Cold storage can effectively turn electricity to cold energy during off peak hours and reduce electricity peak load by supplying cold energy for air conditioning. Solid-liquid phase change rate is seriously encumbered by the relatively-low thermal conductivity of phase change materials (PCMs). A novel fin-foam structure was established to enhance solidification heat transfer and the solidification characteristics were experimentally explored. An experimental system visualizing solid-liquid interface and temperature monitoring was built. The parameters of fin-foam structure, including fin sizes, fin pitch and number were investigated experimentally. Particular attention was paid to justifying the local thermal equilibrium state via measuring temperature on metallic ligament surface and the saturating fluid in pore space. Results showed that inserting fins into metal foam can make a promotional improvement on solidification rate of water by 28.35%. The solid-liquid interface became locally curved after inserting fins. Thermal adhesive and insulation adhesive did not affect the accuracy at pore-scale temperature measurement. Solidification process can be further enhanced through increasing fin width and number rather than fin pitch.

A review of phase change heat transfer in shape-stabilized phase change materials (ss-PCMs) based on porous supports for thermal energy storage

Abstract: Latent heat thermal energy storage (LHTES) uses phase change materials (PCMs) to store and release heat, and can effectively address the mismatch between energy supply and demand. However, it suffers from low thermal conductivity and the leakage problem. One of the solutions is integrating porous supports and PCMs to fabricate shape-stabilized phase change materials (ss-PCMs). The phase change heat transfer in porous ss-PCMs is of fundamental importance for determining thermal-fluidic behaviours and evaluating LHTES system performance. This paper reviews the recent experimental and numerical investigations on phase change heat transfer in porous ss-PCMs. Materials, methods, apparatuses and significant outcomes are included in the section of experimental studies and it is found that paraffin and metal foam are the most used PCM and porous support respectively in the current researches. Numerical advances are reviewed from the aspect of different simulation methods. Compared to representative elementary volume (REV)-scale simulation, the pore-scale simulation can provide extra flow and heat transfer characteristics in pores, exhibiting great potential for the simulation of mesoporous, microporous and hierarchical porous materials. Moreover, there exists a research gap between phase change heat transfer and material preparation. Finally, this review outlooks the future research topics of phase change heat transfer in porous ss-PCMs.

Battery thermal management with thermal energy storage composites of PCM, metal foam, fin and nanoparticle

Abstract: This study aims to analyze the thermal performance of the passive thermal management system (TMS) of the 18,650 lithium-ion battery with application of phase change materials (PCM). To improve performance of TMS, nanoparticles, fins and porous metal foam are used beside the PCM, and their effects on the system performance are compared. The local thermal non-equilibrium (LTNE) model and non-Darcy law are considered to simulate the nano-PCM melting inside the porous media. Numerical results are validated through previously published experimental data and results are presented for two, 4.6 W and 9.2 W, heat generation rates. Sole effects of adding nanoparticles to the PCM, utilizing different numbers of fins, and application of the metal foam on the system performance are scrutinized. Results indicated that the porous-PCM composition performs more efficiently than the nano-PCM and the fin-PCM ones. In addition, ΔTavg, battery parameter is introduced and its variations are analyzed to judge about the effect of each technique to reduce the battery mean temperature. Using the porous-PCM led to 4–6 K reduction in the battery mean temperature with respect to the pure PCM. Moreover, for the porous-PCM composition a delay is observed in the PCM melting initiation time that can adversely affect the performance of battery TMS.

Design of non-uniformly distributed annular fins for a shell-and-tube thermal energy storage unit

Abstract: The shell and tube latent heat thermal energy storage systems are widely recognized as one of the most effective ways to store and utilize solar energy due to their high energy density, constant storage/releasing temperature, structural feasibility and rational price. The application of fins is an effective method to enhance heat exchange through extend the heat transfer surface area. However, there existed strong uneven melting behavior including melting fraction, temperature uniformity in a vertical thermal energy storage unit. To improve its uniformity of melting fraction and temperature, this study designed various conditions for fin pitch and positions in a non-uniform pattern to reduce the inhomogeneity for the melting process. Numerical models were established and validated by the experimental measurement conducted in this study. The effects of fin pitch and position on the thermal performance of the melting process were quantified via analyzing the melting front evolution, temperature and velocity distribution, melting rate and temperature uniformity. Results demonstrated that a 62.8% and 34.4% reduction in full melting time and average temperature difference in the phase change material region was separately obtained for the thermal energy storage unit with non-uniformly distributed annular fins, compared to the original case with uniform fins. Besides, the melting rate uniformity was also improved by 84.7%. The non-uniform design on fin position and pitch give a perspective to enriching the design methods for practical application in engineering.

Configuration, design, and optimization of air-cooled battery thermal management system for electric vehicles: A review

Abstract: A battery thermal management system (BTMS) is arguably the most vital component of an electric vehicle (EV), as it is responsible for ensuring the safe and consistent performance of lithium ion batteries (LiB). LiBs are considered one of the most suitable power options for an EV drivetrain. Owing to lithium's atomic number of three (3) and it being the lightest element of the metals, lithium is able to provide fantastic energy-to-weight characteristics for any lithium-based battery. LiBs are also known for having low self-discharging properties and hence provide long life cycle operation. To obtain a maximum power output from LiBs, it is necessary to critically monitor the operating conditions of LiBs, particularly temperature, which is known to directly affect the performance and life of LiBs. The temperature rise present around LiBs is caused by the heat generation phenomena of lithium ion cells during charge and discharge cycles. In this study, an investigation is made into one of the major categories of a BTMS, used in making the EV powertrain much more efficient and safe. Specifically, this study investigates and reviews air-cooled BTMS techniques (passive and active) and design parameter optimization methods (either via iteration or algorithms) for improving various BTMS design objectives. In particular, this study investigates minimizing the change in temperature among cells (ΔTmax) in a battery pack (BP). The data are classified, and results from recent studies on each method are summarized. It is found that despite features such as extreme simplicity, ease of implementation, and the relatively low cost of naturally air-cooled BTMS, it is almost impossible for the methods to provide adequate cooling conditions for the high energy density LiBs used in EVs. A shift in focus from a naturally air-cooled BTMS to a forced air-cooled BTMS is observed from the amount of studies found on the topics during the time scope of this study. Parameter configuration optimization techniques for the air-cooled BTMS are discussed and classified, and optimization algorithms applied by researchers to improve objectives of the BTMS are identified.

Overview of the Maisotsenko cycle – A way towards dew point evaporative cooling

Abstract: The Maisotsenko Cycle (M-Cycle) is a thermodynamic conception which captures energy from the air by utilizing the psychrometric renewable energy available from the latent heat of water evaporating into the air. The cycle is well-known in the air-conditioning (AC) field due to its potential of dew-point evaporative cooling. However, its applicability has been recently expanded in several energy recovery applications. Therefore, the present study provides the overview of M-Cycle and its application in various heating, ventilation, and air-conditioning (HVAC) systems; cooling systems; and gas turbine power cycles. Principle and features of the M-Cycle are discussed in comparison with conventional evaporative cooling, and consequently the thermodynamic limitation of the cycle is highlighted. It is reported that the standalone M-Cycle AC (MAC) system can achieve the AC load efficiently when the ambient air humidity is not so high regardless of ambient air temperature. Various modifications in MAC system design have been reviewed in order to investigate the M-Cycle applicability in humid regions. It is found that the hybrid, ejector, and desiccant based MAC systems enable a huge energy saving potential to achieve the sensible and latent load of AC in humid regions. Similarly, the overall system performance is significantly improved when the M-Cycle is utilized in cooling towers and evaporative condensers. Furthermore, the M-Cycle conception in gas turbine cycles has been realized recently in which the M-Cycle recuperator provides not only hot and humidified air for combustion but also recovers the heat from the turbine exhaust gases. The M-Cycle nature helps to provide the cooled air for turbine inlet air cooling and to control the pollution by reducing NOx formation during combustion. The study reviews three distinguished Maisotsenko gas turbine power cycles and their comparison with the conventional cycles, which shows the M-Cycle significance in power industry.