Three-dimensional CFD modeling of a planar membrane humidifier for PEM fuel cell systems

A three-dimensional numerical model is developed to investigate and compare the performance of humidifiers with counter-flow and parallel-flow configurations. This model has a set of coupled equations including conservations of mass, momentum, species and energy. The results indicate that in counter-flow humidifier, water and heat transfer is more than that of the parallel-flow that leads to a higher dew point at dry side outlet, consequently, a better humidifier performance. An increase in temperature and a decrease in mass flow rate at dry side inlet lead to a better humidifier performance. However at the low flow rates the humidifier performance does not change a lot by preheating the inlet dry gas. An increase in relative humidity at dry side inlet does not offer any advantage.     

Two dimensional dynamic modeling of a shell-and-tube water-to-gas membrane humidifier for proton exchange membrane fuel cell

Water management is a crucial factor in determining the performance of proton exchange membrane fuel cell (PEMFC) for automotive application. The shell-and-tube water-to-gas membrane humidifier is useful for humidifying the PEMFC due to its good performance. Shell-and-tube water-to-gas membrane humidifiers have liquid water on one side of the tube wall and a dry gas on the other. In order to investigate humidifier performance, a two-dimensional dynamic model of a shell-and-tube water-to-gas membrane humidifier is developed. The model is discretized into three control volumes – shell, tube and membrane – in the cross-sectional direction to resolve the temperature and species concentration of the humidifier. For validation, the dew point temperature of the simulation result is compared with that of experimental data and shows good agreement with only a slight difference. The distribution of humidification characteristics can be captured using the discretization along the air-flow direction. The humidification performance of two different flow configurations, counter and parallel, are compared under various operating conditions and geometric parameters. Finally, the dynamic response of the humidifier at the step-change of various air flow rates is investigated. These results suggest that the model can be used to optimize the inlet flow humidity of a PEMFC.     

Experimental investigation on the heat and water transfer enhancement in a membrane-based air-to-air humidifier at insulation condition

In the present study, a membrane-based air-to-air planar humidifier (MAPH) with baffle-blocked flow channels and a common MAPH are fabricated, tested and compared. These MAPHs are well thermal insulated from their surroundings. Polyoxymethylene (POM) plates with some unique properties such as large tensile and flexural strength, high chemical resistance and high stiffness are used to create channels at dry and humid sides of MAPHs. The obtained findings revealed that the higher heat and water transfer rates and smaller dew point approach temperature (DPAT) in entire tested flow rates occurs in baffle-blocked MAPH. To evaluate the MAPH performance with considering the pressure drop, a dimensionless parameter, performance evaluation criteria (PEC), is introduced. At flow rates less than 1 m³/h, PEC is less than 1, indicating a decline in MAPH performance with considering the pressure drop. In baffle-blocked MAPH using water trap in the inlet of dry side leads to the performance deterioration. Additionally, the increased relative humidity (RH) of humid side inlet causes an increase in DPAT, consequently, the performance deterioration.     

Experimental and thermal analysis of energy efficient novel design wet cooling towers

Waste heat is generally dissipated from process water to atmospheric air in cooling towers. In the present study, a novel design is used to extract more amount of heat without any additional energy input by incorporating secondary ambient air in an induced draft wet cooling tower. In addition, more fresh air is induced in the tower from the rain zone, which increases the effectiveness at any value of the water to air flow rate (L/G ratio). Moreover, tower characteristics, range, and evaporation loss were also increased due to the novel design. It is noteworthy that secondary fresh air increases effectiveness, heat rejection, and tower characteristics by 10.12%, 19.65%, and 26.11%, respectively, and decreases approach by 16.32% at 0.55 L/G ratio, 44°C inlet water temperature, 29.7°C dry bulb temperature, and 18.4°C inlet air wet bulb temperature.     

Comparative study of high concentrating photovoltaics integrated with phase‐change liquid film cooling system

In high concentrating photovoltaic systems, thermal regulation is of great importance to the conversion efficiency and the safety of solar cells. Direct‐contact liquid film cooling technique is an effective way of thermal regulation with low initial investment. Tilt of solar cells is common in concentrating solar systems. An evaluation of direct‐contact liquid film cooling technique behind tilted high concentration photovoltaics was performed using both experimental and computational approaches. In the experiment, deionized water was used as the coolant at the back of simulated solar cells. Solar cell inclination of 0° to 75° with inlet water flow rate of 100–300 L/hour and inlet temperature of 30°C to 75°C were experimentally investigated. A two‐dimensional model was developed using computational fluid dynamics technique and validated by experimental results. The effects of inclination on average temperature, temperature uniformity, and heat transfer coefficient were discovered in this paper. The results indicated that 20° is the optimum angle for liquid film cooling. In addition, optimum inlet width, temperature, and velocity for inclination over 30° are 0.75 mm, 75°C, and 0.855 m/s, respectively.     

Optimum temperatures in a shell and tube condenser with respect to exergy

This paper focuses on evaluation of the optimum cooling water temperature during condensation of saturated water vapor within a shell and tube condenser, through minimization of exergy destruction. First, the relevant exergy destruction is mathematically derived and expressed as a function of operating temperatures and mass flow rates of both vapor and coolant. The optimization problem is defined subject to condensation of the entire vapor mass flow and it is solved based on the sequential quadratic programming (SQP) method. The optimization results are obtained at two different condensation temperatures of 46 °C and 54 °C for an industrial condenser. As the upstream steam mass flow rates increase, the optimal inlet cooling water temperature and exergy efficiency decrease, whereas exergy destruction increases. However, the results are higher for optimum values at a condensation temperature of 54 °C, compared to those when the condensation temperature is 46 °C. For example, when the steam mass flow rate is 1 kg/s and the condensation temperature increases from 46 °C to 54 °C, the optimal upstream coolant temperature increases from 16.78 °C to 25.17 °C. Also, assuming an ambient temperature of 15 °C, the exergy destruction decreases from 172.5 kW to 164.6 kW. A linear dependence of exergy efficiency on dimensionless temperature is described in terms of the ratio of the temperature difference between the inlet cooling water and the environment, to the temperature difference between condensation and environment.     

An experimental study on vapor transport of a hollow fiber membrane module for humidification in proton exchange membrane fuel cells

Water management is key in the optimization of proton exchange membrane fuel cell performance and durability. Humidifiers can be used to provide water vapor to cathode air, ensuring the proper operation of proton exchange membrane fuel cells. In this study, water vapor transport characteristics of hollow fiber membrane modules were investigated in shell-tube humidifiers under isothermal conditions, using two different test jig constructions: a convection jig and a diffusion jig. The mass transfer rate of water vapor was evaluated via the impact of various operating parameters, including temperature, flow rate, pressure, and relative humidity of inlet wet air, flow arrangements, and surface area of the tube side. The result was presented by the water vapor transport rate from wet air flow to dry air flow across the hollow fiber membrane. It was found that humidification performance could be improved with higher operating temperature, flow rate, and relative humidity of inlet wet air, lower pressure, larger membrane surface area, higher convection effect, and substituting co-current with counter-current flow configuration.     

Investigation on cold start of polymer electrolyte membrane fuel cells with different cathode serpentine flow fields

The flow velocity and pressure distribution of the three cathode flow fields are simulated in this study. Larger pressure drop and more rapid flow rate reduce residual water, resulting in minimal ice formation during the cold start process. The simulation results show that the single variable cross section serpentine flow field has the largest pressure drop and the most rapid flow rate.The evolution of the temperature and the segment current density characteristics of three different cathode flow fields during cold start process is studied by printed circuit board technology. The results show that the 2 to 1 serpentine flow field has the best cold start performance and the best current density uniformity when cold start at constant voltage mode above −5 °C. However, the single variable cross section serpentine flow field has the best performance when cold start temperature is below −5 °C. Based on these results, cold start at −30 °C can be realized in 97s by using hot antifreeze liquid.     

Performance analysis of induced draft cooling tower integrated with rotary silica gel mesh

A humid environment decreases the performance of induced draft counter flow cooling towers (IDCFCT) normally used in power plants and process industries. The primary objective of this study is to experimentally analyze the effect of using rotary silica gel mesh (RSGM) on the performance of IDCFCT. Experimental setups of IDCFCT with and without RSGM have been designed and fabricated to perform the comparative analysis. The performance of both cooling towers (CTs) in terms of output temperature, relative humidity of air, range, approach, and effectiveness at different air velocities has been investigated. The results indicate that the outlet air temperature of IDCFCT integrated with the RSGM wheel is 0.5–1.5°C higher than the IDCFCT without the RSGM wheel. IDCFCT with RSGM wheel shows maximum effectiveness of 0.67 at an air velocity of 1.5 m/s and inlet water temperature of 55°C; whereas, for IDCFCT without RSGM wheel, it is only 0.54. In terms of efficiency, RSGM wheel integrated IDCFCT shows an improvement of 24% compared to IDCFCT without RSGM wheel. It has been observed that RSGM-integrated IDCFCT also helps to save water up to 431.7 kg/h at an air velocity of 1 m/s with a water inlet temperature of 35°C. Moreover, the advantage of integrating the RSGM wheel with IDCFCT has been further corroborated in terms of the reduction in the height of the CT obtained with the help of a thermodynamic model.     

Study on a silica gel–water adsorption chiller integrated with a closed wet cooling tower

A silica gel–water adsorption chiller integrated with a closed wet cooling tower is proposed. This adsorption chiller consists of two vacuum chambers, each with one adsorber, one condenser and one evaporator. Vacuum valves were not adopted in this chiller in order to enhance the reliability. A novel heat recovery process was carried out after a mass recovery-like process to improve the coefficient of performance (COP). Integration of the closed wet cooling tower into the chiller could ensure the cleanliness of cooling water circulating in the chiller and also promote the convenient setup of the chiller. A transient one-dimensional mathematical model was adopted to study this adsorption chiller. The simulated results showed that the cooling power and COP were 10.76 kW and 0.51 respectively when the hot water inlet temperature, the chilled water inlet temperature, the air inlet wet bulb temperature and dry bulb temperature were 85, 15, 28 and 30 °C respectively.     

An experimental study on the bubble humidification method of polymer electrolyte membrane fuel cells

A necessary requirement for polymer electrolyte membrane fuel cell (PEMFC) performance is providing sufficient water content in the membrane. The bubble humidifier is the simplest and inexpensive method for PEMFC humidification. In this study, a prototype of bubble humidifier is designed, fabricated, and tested. The effects of water temperature in the reservoir, water level inside the reservoir and inlet air flow on the humidifier performance are investigated. The results show that the outlet air relative humidity decreases (about 6% - 11%) with an increase in the inlet air flow rate from 1 m³ h^?1 to 3 m³ h^?1 at four different water temperatures. The increase in the water temperature and water level inside the reservoir lead to the better humidifier performance. At the water temperature of 20^°C, increasing water level from 5 cm to 7.5 cm has a significant effect on humidifier performance but increasing water level from 7.5 cm to 15 cm does not offer any advantage.     

Periodical shedding of cloud cavitation from a single hydrofoil in high-speed cryogenic channel flow

In order to explain criteria for periodical shedding of the cloud cavitation, flow patterns of cavitation around a plano-convex hydrofoil were observed using a cryogenic cavitation tunnel of a blowdown type. Two hydrofoils of similarity of 20 and 60 mm in chord length with two test sections of 20 and 60 mm in width were prepared. Working fluids were water at ambient temperature, hot water and liquid nitrogen. The parameter range was varied between 0.3 and 1.4 for cavitation number, 9 and 17 m/sec for inlet flow velocity, and −8° and 8° for the flow incidence angle, respectively. At incidence angle 8°, that is, the convex surface being suction surface, periodical shedding of the whole cloud cavitation was observed on the convex surface under the specific condition with cavitation number and inlet flow velocity, respectively, 0.5, 9 m/sec for liquid nitrogen at 192°C and 1.4, 11 m/sec for water at 88°C, whereas under the supercavitation condition, it was not observable. Periodical shedding of cloud cavitation occurs only in the case that there are both the adverse pressure gradient and the slow flow region on the hydrofoil.     

Performance evaluation of mixed convection in an inclined square channel with uniform temperature walls

Numerical analyses were performed for the effect of inclined angle on the mixing flow in a square channel with uniform temperature walls (T_w = 30 °C) and inlet temperature (T₀ = 10 °C). Three-dimensional governing equations were solved numerically for Re = 100, Pr = 0.72 and various inclined angles (from ?90° to 90°). Three-dimensional behavior of fluid in a channel was examined for each angle. Thermal performance was evaluated using the relationship between Nusselt number ratio and pressure loss ratio with and without buoyancy induced flow as a parameter of inclined angles. High heat transfer and low pressure loss region was from ?15° to ?60° in thermal performance using mean Nusselt number ratio.     

Experimental assessment of a direct-contact heat exchanger bubbling hot water in a cooler liquid gallium bath

A direct-contact compact heat exchanger to enhance cooling of hot water, has been manufactured and tested experimentally. Hereby hot water is dispersed into a cooler liquid gallium bath in the form of small water bubbles emanating from 48 holes with 3 mm diameter each, drilled on four horizontal bubbles distribution tubes. Heat transfer limitations posed by gallium's low specific heat have been circumvented by imbedding cooling water tubes within the gallium. Thereby it was possible to maintain gallium at almost 30 °C during water bubbling; slightly above gallium's freezing point. Temperature reduction by about 23 °C was possible for hot water flow with initial temperature of about 60 °C and flow rate of 11.3 g/s when bubbled through such gallium bath that has temperature of about 30 °C and thickness of about only 18 mm. To realize such temperature drop for water using equivalent shell-tube heat exchangers of conventional kinds with 3 mm diameter tubing, a tube length in the range of 70 to 80 cm would be required. Theoretical considerations and empirical correlations dedicated to solid sphere calculations have been used to predict motion and heat transfer events for water bubbles moving through isothermal gallium bath. The computations were extended to include the experimental temperature conditions tested. Computations agree very well with experimental results.     

Design of an air humidifier for a 5 kW proton exchange membrane fuel cell stack operated at elevated temperatures

Air humidification is a crucial issue for superior performance of proton exchange membrane fuel cell (PEM fuel cell) stacks. In this work, an air humidifier is proposed for a 5 kW PEM fuel cell stack working at elevated temperatures, e.g., 90–95 °C. The high temperature coolant exiting the stack is utilized to pre-heat the air in the heat exchanging tubes of the humidifier, and the heated air is humidified with deionized water supplied by a nozzle fixed in a top cavity. Both the tubes and the nozzle are properly designed to ensure sufficient heat transfer and superior atomization. Humidification performance is evaluated under different operation conditions. The nozzle is able to inject well-atomized water with uniform droplet diameter. With the variation of inlet air flow rate, the relative humidity (RH) of the outlet air increases at the beginning, then decreases gradually due to the attenuation of dew point (DP) temperature. However, the humidification performance can be improved when higher temperature deionized water is injected or high temperature coolant is supplied. At a coolant temperature of 95 °C, the outlet air DP temperature is maintained over 80 °C with 25 °C injection water. Moreover, better humidification performance is achieved when the injection water flow rate is controlled according to the working conditions of the stack.     

Performance analysis of a waste heat driven activated carbon based composite adsorbent – Water adsorption chiller using simulation model

This study aims at improving the performance of a waste heat driven adsorption chiller by applying a novel composite adsorbent which is synthesized from activated carbon impregnated by soaking in sodium silicate solution and then in calcium chloride solution. Modeling is performed to analyze the influence of the hot water inlet temperature, cooling water inlet temperature, chilled water inlet temperatures, and adsorption/desorption cycle time on the specific cooling power (SCP) and coefficient of performance (COP) of the chiller system with the composite adsorbent. The simulation calculation indicates a COP value of 0.65 with a driving source temperature of 85 °C in combination with coolant inlet and chilled water inlet temperature of 30 °C and 14 °C, respectively. The most optimum adsorption–desorption cycle time is approximately 360 s based on the performance from COP and SCP. The delivered chilled water temperature is about 9 °C under these operating conditions, achieving a SCP of 380 W/kg.     

Heat pipe heat exchanger for heat recovery in air conditioning

The heat pipe heat exchangers are used in heat recovery applications to cool the incoming fresh air in air conditioning applications. Two streams of fresh and return air have been connected with heat pipe heat exchanger to investigate the thermal performance and effectiveness of heat recovery system. Ratios of mass flow rate between return and fresh air of 1, 1.5 and 2.3 have been adapted to validate the heat transfer and the temperature change of fresh air. Fresh air inlet temperature of 32–40 °C has been controlled, while the inlet return air temperature is kept constant at about 26 °C. The results showed that the temperature changes of fresh and return air are increased with the increase of inlet temperature of fresh air. The effectiveness and heat transfer for both evaporator and condenser sections are also increased to about 48%, when the inlet fresh air temperature is increased to 40 °C. The effect of mass flow rate ratio on effectiveness is positive for evaporator side and negative for condenser side. The enthalpy ratio between the heat recovery and conventional air mixing is increased to about 85% with increasing fresh air inlet temperature. The optimum effectiveness of heat pipe heat exchanger is estimated and compared with the present experimental data. The results showed that the effectiveness is close to the optimum effectiveness at fresh air inlet temperature near the fluid operating temperature of heat pipes.     

Optimization of biodiesel production from waste fish oil

The present study deals with the production of biodiesel using waste fish oil. The research assesses the effect of the transesterification parameters on the biodiesel yield and its properties, including temperature (40–60 °C), molar ratio methanol to oil (3:1–9:1) and reaction time (30–90 min). The experimental results were fitted to complete quadratic models and optimized by response surface methodology. All the biodiesel samples presented a FAME content higher than 93 wt.% with a maximum, 95.39 wt.%, at 60 °C, 9:1 of methanol to oil ratio and 90 min. On the other hand, a maximum biodiesel yield was found at the same methanol to oil ratio and reaction time conditions but at lower temperature, 40 °C, which reduced the saponification of triglycerides by the alkaline catalyst employed. Adequate values of kinematic viscosity (measured at 30 °C) were obtained, with a minimum of 6.30 mm²/s obtained at 60 °C, 5.15:1 of methanol to oil ratio and 55.52 min. However, the oxidative stability of the biodiesels produced must be further improved by adding antioxidants because low values of IP, below 2.22 h, were obtained. Finally, satisfactory values of completion of melt onset temperature, ranging from 3.31 °C to 3.83 °C, were measured.     

Experimental study of saturated flow boiling heat transfer in an array of staggered micro-pin-fins

This study concerns water saturated flow boiling heat transfer in an array of staggered square micro-pin-fins having a 200 × 200 μm² cross-section by a 670 μm height. Three inlet temperatures of 90, 60, and 30 °C, six mass velocities for each inlet temperature, ranging from 183 to 420 kg/m² s, and outlet pressures between 1.03 and 1.08 bar were tested. Heat fluxes ranged from 23.7 to 248.5 W/cm². Heat transfer coefficient was fairly constant at high quality, insensitive to both quality and mass velocity. Heat transfer was enhanced by inlet subcooling at low quality. Possible heat transfer mechanism was discussed.     

Experimental study of impingement spray cooling for high power devices

An experimental study of a closed-loop impingement spray cooling system to cool a 1 kW 6U electronic test card has been conducted. The system uses R134a as working fluid in a modified refrigeration cycle. The spray from four vapor assisted nozzles is arranged to cover a large ratio of the heated area of the card. Investigations are currently focused on effects of mass flow rate, nozzle inlet pressure and spray chamber pressure. Experimental results are promising with a stable average temperature of around 23 °C being maintained at the heated surface, and maximum temperature variation of about 2 °C under suitable operating conditions. Heat transfer coefficients up to 5596 W/m² K can be achieved with heat flux input around 50,000 W/m² in this study. It is found that cooling performance improved with increasing mass flow rate, nozzle inlet pressure and spray chamber pressure, whereas uniformity of the heated surface temperature can only be improved with higher mass flow rate and nozzle inlet pressure. The mechanisms for the enhanced performance are also presented.