Performance prediction of an improved air-cooled steam condenser with deflector under strong wind

For an air-cooled steam condenser (ACSC), environmental wind can cause a large flow rate reduction in the axial fans mainly near the windward side of the air-cooled platform due to cross-flow effects, resulting in a heat transfer reduction. This leads to an increase of turbine back pressure, and occasional turbine trips occur under extremely gusty conditions. A new method is proposed in this paper to remove the strong wind effect by adding deflecting plates under the air-cooled platform, which contributes to forming a uniform air mass flow rate in the axial fans by leading enough cooling air to the fans in the upwind region. Numerical simulation is made of the thermal-flow characteristics and heat transfer performance of the improved ACSC with deflectors. A heat exchanger model is used for simulating the flow and heat transfer in the ACSC, in which the heat exchanger is simplified to a porous medium and all flow losses are taken into account by a viscous and an inertial loss coefficient. A fan model is used for reaching the flow condition at the heat exchanger inlet with the actual performance curves of the fan. It is found that the improved ACSC with deflector shows a significant enhancement in both the cooling air mass flow rate and the heat rejection rate compared with the conventional ACSC. The higher the wind speed is, the larger the heat transfer enhancement of the improved ACSC is. The effect of the plate inclination is also investigated, and the inclination angle of 45° is found to be the optimum value for the arrangement of the deflector.     

Performance prediction of proton exchange membrane fuel cell engine thermal management system using 1D and 3D integrating numerical simulation

The cooling system of proton exchange membrane fuel cell (PEMFC) engine was simulated by 1D and 3D collaborative simulation method. Firstly, the resistance characteristics of the flow channel are obtained by simulating the airside flow model. A three-dimensional simulation model including dual fans and radiator is also established to simulate the airflow distribution. The one-dimensional simulation model of 30 kW PEMFC engine cooling system that are mainly composed of a thermostat, water pump, and fan and radiator model is established. Secondly, the heat dissipation performance of the cooling system is calculated by using the coupled simulation model. It is found that the simulation results of the amount of heat transferred are in good agreement with the experimental data by compromising, which proves that the model is reasonable. Finally, the thermal performance of the extreme operating conditions of the PEMFC system is simulated by means of a simulation model. By monitoring the flow of the pump and the fan speed, we can maintain the stack internal heat balances, so that the stack efficient and stable operation. The results demonstrate that the 3D simulation can get the distribution of fluid flow more accurately, while the simulation time of 1D thermal system is short and can guide the matching of heat transfer parts quickly.     

Development of a radial-flow multiple magnetically coupled fan system with one piezoelectric actuator

In recent years, piezoelectric fans and their feasibility for use in cooling electronic devices have been widely studied. However, there are few studies that address using a single piezoelectric actuator to generate radial air flow. In this study, a radial-flow multiple fan system (RMFS) was developed for the thermal management of high power LEDs. This system only used one piezoelectric actuator and a magnetic repulsive force to activate up to 20 fans, which featured low power consumption and a large cooling area. The RMFS was mounted in a circular heat sink to evaluate its thermal performance. To find the optimal design for the RMFS, the influence of some geometric parameters was investigated. The performance of different designs was compared with a commercially available axial fan. The results showed that design E had the best thermal performance among the designs because of its relatively large frequency and amplitude. The thermal resistance and percentage improvement under a 35 W heat flux were 0.86 K/W and 36.9%, respectively. In addition, a coefficient of performance (COP) was defined. The COP of design E was approximately 3.7 times that of the rotary fan. For the power consumption aspect, the RMFS is more efficient than the rotary fan.     

Air cooling for a large-scale motor

This article experimentally and numerically investigates the thermal performance of a large-scale motor with a capacity of 2350 kW. The large-scale motor consists of a centrifugal fan, two axial fans, a shaft, a stator, a rotor and a heat exchanger with 637 cooling tubes. The test rigs are set up to measure the performance of the fans and the temperature distributions of the motor. The models of the fan and motor have been implemented in a Fluent software package to predict the flow and temperature fields inside the motor. The calculated results show good agreement with the measured data. In order to improve the motor thermal performance, several methods have been adopted, which are aiming to enhance the fan performance by changing the geometry, to redesign a new heat exchanger with guide vanes, and to optimize the distance between the axial fans. The modified design can decrease the temperature rise by 6 °C in both the stator and rotor.     

Effect of cross-flow on the performance of air-cooled heat exchanger fans

In large-scale applications such as arrays of axial fans in air-cooled heat exchanger systems, edge–proximity and wind-induced cross-flow may decrease the flow through some fans by causing the flow to enter them at off-axis angles. In this study, such off-axis inflows were introduced by inserting inlet pipe sections between the plenum chamber of a standard test facility and one of three different scale model test fans of 1542 mm diameter. Fan power consumption turned out to be completely independent of off-axis inflow angle up to 45°. Fan total-to-total pressure rise was found to be independent of off-axis inflow angle, and the decrement in fan pressure rise was equal to the dynamic pressure based on the cross-flow velocity component at the fan inlet. Analysis showed that for model fans to represent the cross-flow behaviour of their prototypes, they should have the same ratio of dynamic pressure to pressure rise, and the same dimensionless characteristic slope at their operating points. The performance of a row of fans operating at off-axis inflow conditions representing a cooling system was well predicted by a simple model assuming that the fans farther from the edges induce cross-flows over the fans closer to the edges.     

Heat transfer and performance characteristics of axial cooling fans with downstream guide vanes

This study examines experimentally the effect of stators on the performance and heat transfer characteristics of small axial cooling fans. A single fan impeller, followed by nine stator blades in the case of a complete stage, was used for all the experimental configurations. Performance measurements were carried out in a constant speed stage performance test rig while the transient liquid crystal technique was used for the heat transfer measurements. Full surface heat transfer coefficient distributions were obtained by recording the temperature history of liquid crystals on a target plate. The experimental data indicated that the results are highly affected by the flow conditions at the fan outlet. Stators can be beneficial in terms of pressure drop and efficiency, and thus more economical operation, as well as, in the local heat transfer distribution at the wake of the stator blades if the fan is installed very close to the cooling object. However, as the separation distance increases, enhanced heat transfer rate in the order of 25% is observed in the case of the fan impeller.     

Cooling of a permanent magnet electric motor with a centrifugal impeller

This paper presents a thermal fluid analysis on the air cooling of a permanent magnet electric motor with a centrifugal impeller. A numerical model is developed for the heat transfer and fluid flow process. The flow rates of the cooling air are also experimentally measured. The agreement between the numerical model prediction and experimental data is reasonably good. Detailed structures of the cooling flow are presented. Convection heat transfer on the surface of the armature is quantified. Comments on the application of the motor architecture are given. Design modifications are proposed for performance improvements.     

横流式冷却塔换热模型与影响因素研究

根据Merkel的冷却塔传热传质理论,推导了适应于横流式冷却塔的换热模型,通过理论模型正交试验和实测数据因子相关性分析,研究了横流式冷却塔换热性能的影响因素。实测数据因子相关性分析结果表明,风量对横流式冷却塔换热性能影响程度最小,与理论模型正交实验结果存在一定的差异。运行时应保证横流式冷却塔进水流量的分布均匀,才能更有效的利用换热面积,提高横流式冷却塔换热效率。     

Model for predicting performance of cooling fans for thermal design of electronic equipment (Modeling and evaluation of effects from electronic enclosure and inlet sizes)

This study describes the performance of cooling fans in terms of the P–Q curve and the maximum flow rate under various environmental conditions. It focuses on the relationship between fan performance and configuration factors such as the electronic enclosure. The presence of an enclosure wall increased the pressure characteristic of the fan performance. The presence of a narrow inlet decreased the flow rate. When the inlet area of the enclosure became smaller than twice the fan flow area, the flow rate was decreased. The maximum flow rate depended on the ratio of the inlet area to the fan flow area. A model for predicting pressure rise and flow rates in the enclosure is proposed. The model is used in a thermal analysis of a PCB model set in an enclosure. © 2011 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library ( wileyonlinelibrary.com/journal/htj ). DOI 10.1002/htj.20347     

Forced air cooling for CPU modules with high heat dissipation

We describe forced air cooling, based on two new concepts, for CPU modules with high heat dissipation. The first concept is a slit jet flow system, and the second is an impingement duct flow system. These systems are designed for electronic equipment with multiple circuit boards (CPU modules), on which the CPUs have high heat dissipation and the SRAMs have low heat dissipation, loaded in three dimensions and in parallel. The slit jet flow system is very simple compared with the impingement jet flow system, which has a complicated comb‐style impingement jet. The slit jet system includes an air duct with slit orifices and an axial fan upstream from the circuit boards. The impingement duct system consists of an air duct and a heat sink with a fan, which is attached to the CPU. This system also has slit orifices and an axial fan upstream or downstream from the circuit boards. For electronic equipment with each of these flow systems installed, the increases in external thermal resistance for the CPU and SRAMs were measured after stopping one of the cooling fans or the CPU's micro cooling fan. The results showed that the slit jet and impingement duct flow systems provide good cooling performance and redundancy. © 2002 Wiley Periodicals, Inc. Heat Trans Asian Res, 31(3): 226–236, 2002; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.10031     

Thermal performance of an evaporatively cooled building: Parametric studies

A single-storey office building in a hot and dry climate is modelled for evaporative cooling. The counterflow cooling tower is modified to precool the tower inlet air by the tower exit air in a heat exchanger. Centralized evaporative air cooling, using the modified cooling tower, and roof evaporative cooling are considered to provide comfortable living conditions in the space. The thermal performance of such a building is analysed for various operating parameters. The study indicates that centralized evaporative air cooling is feasible, to maintain near-perfect comfort conditions in hot and dry climates. Modified cooling tower and roof evaporative cooling further enhance the scope of evaporative cooling for comfort applications.     

Cool Thermal Discharge by Melting Ice and Producing Chilled Air

This article deals with the time histories of the air flow rate required and the temperature of chilled air produced during cooling of air by melting ice with specified heat flux at the free liquid surface. The equations for estimating the required mass flow rate of air to maintain a specified heat flux removal of cool thermal discharge by direct contact of air with melting ice to produce chilled air were derived from an analysis of the heat transfer coupled with the moving boundary. Numerical examples are illustrated in which the inlet air temperatures were either specified or varied with time to simulate practical systems.     

Unsteady flow condition of contra-rotating small-sized axial fan

Small-sized axial fans are used as air cooler for electric equipments.But there is a strong demand for higher power of fans according to the increase of quantity of heat from electric devices.Therefore,higher rotational speed design is conducted,although,it causes the deterioration of efficiency and the increase of noise.Then,the adoption of contra-rotating rotors for the small-sized axial fan is proposed for the improvement of performance.In the case of contra-rotating rotors,it is necessary to design the rotor considering the unsteady flow condition of each front and rear rotor.In the present paper,the fan performance of the contra-rotating small-sized axial fan with 100mm diameter at a designed and a partial flow rates is shown,and the unsteady flow conditions at the inlet and the outlet of each front and rear rotor are clarified with unsteady numerical results.Furthermore,the relation between the performance and the unsteady flow condition of the contra-rotating small-sized axial fan is discussed and the methods to improve the performance are considered.     

Thermal analysis and management of proton exchange membrane fuel cell stacks for automotive vehicle

The thermal management of a proton exchange membrane fuel cell (PEMFC) is crucial for fuel cell vehicles. This paper presents a new simulation model for the water-cooled PEMFC stacks for automotive vehicles and cooling systems. The cooling system model considers both the cooling of the stack and cooling of the compressed air through the intercooler. Theoretical analysis was carried out to calculate the heat dissipation requirements for the cooling system. The case study results show that more than 99.0% of heat dissipation requirement is for thermal management of the PEMFC stack; more than 98.5% of cooling water will be distributed to the stack cooling loop. It is also demonstrated that controlling cooling water flow rate and stack inlet cooling water temperature could effectively satisfy thermal management constraints. These thermal management constraints are differences in stack inlet and outlet cooling water temperature, stack temperature, fan power consumption, and pump power consumption.     

热管用于笔记本电脑智能温控散热的分析

随着笔记本电脑性能的不断提升,传统的单一风冷散热已经满足不了要求,传热性能优越的热管便应用于笔记本电脑散热。分析了热管用于智能温控散热系统的传热机理,并建立了传热模型．分析了用于笔记本散热的热管的热阻和总传热系数,结合实例进行了定量计算。计算结果表明热管配合智能温控风扇,能很好满足笔记本散热的要求。     

Measurement and prediction of the cooling characteristics of a generalized vibrating piezoelectric fan

Piezoelectric fans are thin elastic beams whose vibratory motion is actuated by means of a piezoelectric material bonded to the beam. These fans have found use as a means to enhance convective heat transfer while requiring only small amounts of power. The objective of the present work is to quantify the influence of each operational parameter and its relative impact on thermal performance. Of particular interest are the vibration frequency and amplitude as well as the geometry of the vibrating cantilever beam. The experimental setup consists of a piezoelectric fan mounted normal to a constant heat flux surface. Temperature contours on this surface captured via an infrared camera are used to extract the forced convection coefficient due to the fluid motion generated from the fan. Different fans, with fundamental resonance frequencies ranging from 60 to 250 Hz, are considered. Results show that the performance of the fans is maximized at a particular value of the gap between the fan tip and the heated surface. It is found that when a fan operates at this optimum gap, the heat transfer rate is dependent only on the frequency and amplitude of oscillation. Correlations based on appropriately defined dimensionless parameters are developed and found to successfully predict the thermal performance across the entire range of fan dimensions, vibration frequency and amplitude. An understanding of the dependence of thermal performance on the governing variables allows for improved design of piezoelectric fans as a method of enhancing heat transfer.     

The use of CFD for predicting and optimizing the performance of air conditioning equipment

This paper reports on the use of CFD for predicting and improving the performance of a rooftop AC unit. The current work considered the hydrodynamic and thermal fields on the air flow side of the unit with exact modeling of fans and heat exchangers. This is in addition to predicting condensation on cooling coils. Because only the air flow side is considered, the evaporator and condenser compartments are decoupled and the solution in each section is established separately. In the evaporator compartment the flow is solved as a two-phase flow (gas and liquid) with the gas phase being composed of two species (dry air and water vapor). In the condenser section however, the flow is treated as a single phase flow. The exact modeling of heat exchangers and fans increased the grid size and computational cost, but resulted in realistic results and reliable model. A total of 31 million control volumes are used to model the evaporator and condenser sections. Results indicate the presence of several recirculation zones in the evaporator compartment. Sensible and latent cooling capacities for several design conditions predicted by the model are in close agreement with available experimental data. The differences between the total capacities predicted by the model in the evaporator section and those reported experimentally are within 2.7% for all cases considered. Predictions in the condenser section resulted in a load that is only 0.00136% different than the one calculated using experimental data. To improve the performance of the unit, six different modified designs of the evaporator coil are developed and tested. The newly modified designs are based on changing the coil inclination angle and/or number of fins per unit length for the same coil height and surface area. One of the designs resulted in 6.18% decrease in the cooling capacity, while the remaining modifications increased the cooling capacity by values ranging between 2.17% and 8.6%.     

Effect of synchronized piezoelectric fans on microelectronic cooling performance

This study reports on the influence of dual vibrating fans on flow and thermal fields through numerical analyses and experimental measurements. Two piezoelectric fans were arranged face to face and were vertically oriented to the heat source. 3D simulation was performed with FLUENT and ABAQUS with the use of code coupling interface MpCCI to calculate the velocity and temperature distribution on the horizontal hot plate. The fans' motion was described as deformable parts by ABAQUS at their first mode vibration. The effects of vibration phase difference between the fans corresponding to in-phase (Φ = 0°) and out-of-phase (Φ = 180°) vibrations were explored in terms of transient temperature and flow fields. The purpose is to enhance heat dissipation from the microelectronic component. Comparison with the performance of a single fan is made to assess the significance of the additional fan on thermal performance. Good comparison results were achieved through accurate modeling of the most important features of the fans and through heat transfer. Computed results show that the single fan enhanced heat transfer performance within approximately 2.3 times for the heated surface. By contrast, the dual fans enhanced heat transfer performance within approximately 2.9 for out-of-phase vibration (Φ = 180°) and 3.1 for in-phase vibration (Φ = 0°).     

On the Improvement of the Poor Heat Transfer Lee-Side Regions of Square Cross-Section Ribbed Channels

Heat transfer and flow characteristics of six ribbed channels of square cross section having different rib structure are computed with the objective of improving heat transfer in the lee-side of the ribs. Six ribs are installed on the bottom walls of each channel. The rib pitch-to-height ratio (P/e) is 10. Details of the turbulent flow structure, temperature fields, local heat transfer coefficients, flow friction coefficients, normalized heat transfer rates, and normalized friction factors are reported. The simulations use the v²f turbulence model and inlet Reynolds number range of 8,000 to 24,000. A uniform heat flux is appropriately applied on all surfaces. The heat transfer performances features of the ribbed channels of various designs are evaluated and compared. A case with an inclined lee-side structure having an inclination angle of 160° yields the highest Nusselt number and friction factor, about 4.6%–6.4% higher than those with rectangular ribs, and 7.1%–9.0% higher heat transfer when the heated-surface area is considered. Increased pressure drop is kept within certain limits when considering the balance between cooling effectiveness and pressure loss for the comparisons. Though having the best heat transfer, the case with the inclined back-wall geometry of the ribs does not present the better overall thermal performance due to the higher friction. The heat transfer enhancement is more prominent when improvements of the poor heat transfer regions downstream of the rib are computed with the surface area change excluded. A conclusion to be drawn is that lee-side improvement of heat transfer can be effected with suitable design of the rib downstream side. This finding can be applied to improvement of turbine airfoil cooling.     

Influence of fan setting height on the cooling performance of a plate‐fin‐type heat sink with microprocessor

The cooling performance of a plate‐fin‐type heat sink equipped with a cooling fan was investigated experimentally. The heat sink was 80 mm long, 43 mm wide, and 24 mm in height (including the 4‐mm‐thick base). The cooling fan was 40 × 40 × 15 mm and was set to direct the air flow vertically in the downstream half of the heat sink. We focused on the influence of the height (which varied from 5 to 20 mm) that the fan was set at, on the heat transfer coefficient of the heat sink. The maximum value of the heat transfer coefficient was achieved at a setting height of 5 mm. At this height, the volumetric heat transfer coefficient was 1.8 times as high as that in a parallel flow under the same fan power. This result indicates that the cooling performance of heat sinks with a cooling fan can be improved by using this kind of compact structure. © 2001 Scripta Technica, Heat Trans Asian Res, 30(6): 512–520, 2001