Thermal analysis of tilted wick solar distillation-cum-drying system

The performance of the designed tilted wick solar water distillation-cum-drying unit has been tested at water flow rates of 50 and 65 ml/min in the distillation unit. Effect of water flow rates on the heat transfer coefficients of distillation and drying unit for drying ginger has been evaluated. The energy and exergy efficiency of the distillation system have also been evaluated. Average distillates of 2.36 and 2.2 l/m² were collected from the tilted wick solar still at flow rates of 50 and 65 ml/min, respectively. Large variation in convective and evaporative heat transfer coefficients of distillation unit has been observed at given water flow rates. Water flow rate in the distillation unit significantly affects the performance of the drying unit. Average convective heat transfer coefficients of 6.56 and 3.75 W/m^{2 o}C have been observed for drying ginger at flow rates of 50 and 65 ml/min, respectively. Energy and exergy efficiency of the distillation unit have been found to be nearly 19% and 0.9%, respectively. Experimental uncertainty has also been evaluated for distillation and drying units. The distillate cost for the developed distillation-cum-drying unit is calculated as $0.03729/l along with dried ginger of about 2.5 kg/m²/day.     

Testing and performance analysis of a hollow fiber-based core for evaporative cooling and liquid desiccant dehumidification

In this study, an innovative heat and mass transfer core is proposed to provide thermal comfort and humidity control using a hollow fiber contactor with multiple bundles of micro-porous hollow fibers. The hollow fiber-based core utilizes 12 bundles aligned vertically, each with 1,000 packed polypropylene hollow fibers. The proposed core was developed and tested under various operating and ambient conditions as a cooling core for a compact evaporative cooling unit and a dehumidification core for a liquid desiccant dehumidification unit. As a cooling core, the fiber-based evaporative cooler provides a maximum cooling capacity of 502 W with a wet bulb effectiveness of 85%. As a dehumidification core and employing potassium formate as a liquid desiccant, the dehumidifier is capable of reducing the air relative humidity by 17% with an overall dehumidification capacity of 733 W and humidity effectiveness of 47%. Being cheap and simple to design with their attractive heat and mass transfer characteristics and the corresponding large surface area-to-volume ratio, hollow fiber membrane contactors provide a promising alternative for cooling and dehumidification applications.     

Numerical studies on the thermal regimes of the horizontal tube falling film evaporation under varying feeder height

ABSTRACT

This paper discusses about the effect of feeder height and heat flux on the heat transfer characteristics of horizontal tube falling film evaporation in the thermal regimes. In order to investigate this, a two- dimensional CFD model was developed to perform simulation and results were compared and validated with published data available in the literature. Heat transfer co-efficients in the thermal regimes were determined from the CFD simulation and the results were recorded, analyzed and validated with the mathematical models available in the literature. The novelty of the current study is to predict the commencement of the fully developed thermal region over the tube from the simulation model under varying feeder height and heat flux. An effort was also made to measure the liquid film thickness around the tube from the CFD model in the thermal regimes. It is observed that angle of thermally developing region contracts and fully developed thermal region extends with the increase of the feeder height and heat flux. It is observed from the study that increase of heat flux by 10 kW/m² resulted in increase of heat transfer co-efficient value by 10–12% average in thermally developing region and 12–15% average in fully developed region. Thinnest liquid film thickness observed between 85 and 127°angle. Shifting of thinnest region of liquid film upward from the mid tube with the increase of the feeder height and heat flux is noted.     

Photovoltaic-thermal (PV/t) panel to minimize electrical and air conditioning energy consumption of a typical office in Beirut

A combined photovoltaic–thermal (PV/t) panel is proposed to produce simultaneously electricity and heat from one integrated unit. The unit utilizes effectively the solar energy through achieving higher PV electrical efficiency and using the thermal energy for heating applications. To predict the performance of the PV/t at a given environmental conditions, a transient mathematical model was developed. The model was integrated in a heating application for a typical office space in the city of Beirut to provide the office needs for electricity, heating during winter season, and dehumidification and evaporative cooling during the summer season. To minimize the yearly office energy (electrical and heat) needs, the PV/t panel cooling air flow rate and the dehumidification regeneration temperature were determined for opimal unit operation. Thermal energy savings of up to 85% in winter and 71% in summer were achived compared to conventional systems at a payback period of 8 years for the panels.     

Experimental investigation of a solar water distillation-cum-drying unit

In this communication, a new design of solar-energy-based water distillation cum drying unit with parabolic reflector has been designed, fabricated, and tested. Bitter gourd and potato slices are chosen as a drying commodity. Thermal performance of the developed system has been evaluated based on the experimental results and using linear regression analysis. Heat transfer coefficients (convective, evaporative, and radiative) for solar distillation system have been observed to be 2.48–4.09, 13.25–52.38, and 8.75–9.66 W/m²°C, respectively. Overall thermal efficiency and exergy efficiency for the distillation system has been found to be 18.77% and 1.2%, respectively. The convective heat transfer coefficient for potato slices are observed higher for initial hours and decreases as the day progresses. The average convective heat transfer coefficients for bitter gourd and potato slices have been observed as 2.18 and 5.04 W/m²°C, respectively. Experimental error in terms of percent uncertainty for bitter gourd and potato slices are found to be 42.93% and 37.06%, respectively. The present design of solar distillation and drying in a single unit could be beneficial for the development of remote, arid, and rural areas.     

Experimental energy and exergy analyses of a discharging heat exchanger for a small hot-oil domestic storage tank

Experiments are described to investigate the thermal performance of a discharging heat exchanger for a small storage tank filled with oil. Experimental results are presented in terms of the discharging energy rates (power) and the discharging exergy rates for low (~4 ml/s) and high discharging flow rates (~8 ml/s). Water heating energy rates, which are respectively maximized at approximately 600 W and 1200 W at low and high flow-rate discharging, are found to be higher than the discharging energy rates, which are respectively maximized at 450 W and 900 W. These results indicate that the energy rates do not accurately evaluate the thermal performance of the discharging heat exchanger since the energy heating rate of the water is greater than that for the oil that heats it, which is thermodynamically inconsistent. The energy rates should thus be used with caution when the thermal performance of the heat exchanger is evaluated. Water heating exergy rates, which are respectively maximized at approximately 45 W and 130 W at low and high flow-rate discharging, are generally smaller than the discharging exergy rates, which are respectively maximized at 65 W and 170 W. Exergy rate results are thus more consistent in the physical process of water heating, and an exergy factor is suggested as a proper measure for evaluating the performance of the discharging heat exchanger. The maximum value of the exergy factor is found to increase from 0.15 at low flow rates to a maximum value of approximately 0.19 at high flow rates. This implies that to extract more energy from a storage tank to a discharging heat exchanger, the flow rate has to be high, which is consistent with the physical process of heating water faster to higher temperatures. The exergy factor can thus be used as a design parameter for discharging heat exchangers.     

Thermodynamic coupling characteristics in hybrid (dry/wet) cooling system

A hybrid cooling system consisting of both a dry section and wet section is proposed in this paper as a means to conserving energy and water by combining the benefits of both dry and wet cooling modes. A new thermodynamic coupling characteristics computing model was established to identify the best combination of dry and wet cooling subsystems in the hybrid tower throughout year-round operation based on its air thermodynamic state under the “no plume” principle. A hybrid cooling tower in Inner Mongolia, China, consisting of an elliptical tube heat exchanger with rectangular fins and counter-flow wet packing, was designed as an example under the no plume principle. The minimum number of heat exchanger units in service and the corresponding thermodynamic operating parameters were obtained under a year-round operation. The tower exhibited notable advantages in regards to water conservation compared to the traditional evaporative cooling tower at an estimated yearly savings of 3.74 × 10⁷ kg water.     

Effect of pipes in heat pump system on electric vehicle energy saving

ABSTRACT

Refrigerant pressure drop and temperature change in pipes are normally ignored in the thermodynamic analysis of traditional vehicle air conditioning system, this will result in serious errors. In this Paper, pressure drop and temperature difference are simulated in different pipes of electric vehicle (EV) heat pump system to analysis the effects of pipes in the actual EV heat pump system. The results indicate that the greater the mass flow, the faster pressure drop increases, the temperature difference decreases. Pressure drop of saturated liquid refrigerant is smaller than that of saturated gas refrigerant at the same saturation pressure and mass flow rate. The higher the refrigerant pressure (no phase change), the slower pressure drop decreases, the faster the temperature difference decreases. Pressure drop decreases with the increment of bending angle of the pipe. For EV heat pump system, suitable valves and less branches are helpful for energy saving of the system. Shortening the pipe between compressor and condenser can reduce temperature change obviously. Pressure drop per unit length in the pipe between evaporator and compressor is large especially in heating mode because of lower refrigerant density. It even reaches to over 100 times of that in the pipe between condenser and throttle valve in heating mode and has negative effects on the performance of the system. If the evaporator is closer to the compressor and the number of branches is less, then pressure drop will decrease a lot, which will be advantageous for energy saving of the heat pump system.     

Heat dissipation characteristic in the intake grille and radiator of a fuel cell vehicle

ABSTRACT

In this study, a three-dimension (3D) computational model was proposed to investigate the flow and heat transfer characteristics of the intake grilles of two different fuel cell vehicles. The models of the intake grilles were constructed according to the actual sizes of two vehicles, namely, Roewe 950 and Toyota Mirai, considering the heat dissipation unit to simplify the heat transfer model of the vehicle. The results showed that relative to Roewe 950, Mirai intake air flow rate was approximately 10% higher, the heat transfer capacity was approximately 7% higher, and the intake grille area was larger. The coolant outlet temperature of Mirai was lower than that of Roewe 950, which was beneficial for the long term and stable operation of a fuel cell. This comparative study provided guidance for the intake grille and radiator design of fuel cell vehicles. The only difference between fuel cell vehicles on the market and conventional vehicles was that in the former, the internal combustion engine was replaced with a fuel cell stack, which had insufficient heat transfer capacity because of the reducing temperature difference. Increasing the intake grille area and the heat exchange capacity of the radiator were the key issues for the development of fuel cell vehicles. In this study, an optimal window opening angle of the radiator fin of 23° provided a maximal heat transfer coefficient.     

Thermal Modeling and Performance Evaluation of Photovoltaic Thermal (PV/T) Systems: A Parametric Study

Conventional solar photovoltaic (PV) module converts the light component of solar radiation into electrical power, and heat part is absorbed by module increasing its operating temperature. Combined PV module and heat exchanger generating both electrical and thermal powers is called as hybrid photovoltaic/thermal (PV/T) solar system. The paper presents the design of a PV/T collector, made with thin film PV technology and a spiral flow absorber, and a simulation model, developed through the system of several mathematical equations, to evaluate the performance of PV/T water collectors. The effect of various parameters on the thermal and electrical efficiency has been investigated to obtain optimum combination of parameters. Finally, a numerical simulation has been carried out for the daily and annual yield of the proposed PV/T collector, and comparison with a standard PV module is discussed.     

Modeling of Fluid Flow and Heat Transfer in a Copper Based Heat Sink Application

With the trend towards increasing the speed of processors in smaller sized of computers, there has been considerable interest in heat sink technologies with higher levels of performance and further miniaturization. This work addresses the fundamental heat transfer augmentation question of how to design a copper-based heat sink, when the overall dimensions of the bottom plate or fan are specified. A three-dimensional finite-volume model has been developed and applied to investigate flow and conjugate heat transfer in the copper-based heat sink. The model was produced with the commercial program FLUENT, which allows this nonlinear, highly turbulent problem to be simulated using the k-ε turbulence model. The theoretical model developed is validated by comparing the model predictions with available experimental data. The thermal performance and temperature distribution for the heat sink were analyzed and a procedure for optimizing the geometrical design parameters based on less space occupation and more efficient heat transfer coefficient is presented. Several design examples with different types of cooling methods and manufacturing processes have been analyzed. The reliability and effectiveness in heat spreading of those has been compared. It has been shown that the copper-based heat sink with louvered fins (case No.3) has an optimum design configuration.     

An Experimental Study of Heat Pipe Performance Using Nanofluids

Heat pipe cooling is widely used in computer processors. Advances in microprocessor technology have resulted in reduced heat transfer surface area. Maintaining an efficient cooling process is therefore challenging. The main goal of this experimental study is to perform a parametric study on heat pipe performance using nanofluids. Nanofluids of 1 and 3 vol% of alumina nanoparticles of 20–50 nm diameters in deionized water versus deionized water as a base fluid were considered in the present study. The nanofluids are prepared in our laboratory using two-step method. The nanofluids thermal properties are measured to confirm the properties enhancement that could indicate a corresponding performance enhancement of the heat pipe. A 10 mm inner diameter, 200 mm long brass tube with 50 mm long evaporator, and 50 mm long water cooled condenser were used. Heat pipe wall temperature is reduced with nanofluids as is the temperature difference between the evaporator and condenser. The thermal diffusivity of the nanofluids is increased by 10%. The pipe pressure in case of deionized water was higher than the corresponding one for the nanofluids by 20–32%.     

Frequency Control Characteristics of a Direct-Mode Untreated Sewage Source Heat Pump in the Heating Mode

An untreated sewage source heat pump system directly makes use of the urban raw sewage instead of that treated by a sewage plant. At present In China, most systems adopt the indirect mode for avoiding the harmful effect of the sewage on the heat pump unit. In this article a direct-mode untreated sewage source heat pump system, which shows less theoretical analysis of the mathematical model, is theoretically designed and analyzed to simulate the system dynamic characteristics in the heating mode. The results show that the system COP changes from 4.1 to 3.4 and the heating capacity from 9.5 to 15.3 kW when the sewage inlet temperature is 12°C and the frequency increases from 18 to 32 HZ. The condenser heat-transfer coefficient increases with the frequency reducing while the change trend of evaporator heat-transfer coefficient is the opposite. The highest values of them are 303 and 1617 W.m^?2?K^?1, respectively. The frequency control simulation supplies the operation adjustment with theoretical instructions and some reference values.     

Modeling of Flow Field and Heat Transfer in a Copper-Base Automotive Radiator Application

This paper describes a methodology used for designing louvered fins. Louvered fins are commonly used in many compact heat exchangers to increase the surface area and initiate new boundary layer growth. Detailed measurements can be accomplished with computational models of these louvered fins to gain a better understanding of the flow field and heat distribution. The particular louver geometry studies for this work have a louver angle of 23° and fin count of 17 fpi.

The flow and heat transfer characteristics for three-dimensional mixed convection flows in a radiator flat tube with louvered fins are analyzed numerically. A three-dimensional model is developed to investigate flow and conjugate heat transfer in the copper-based car radiator. The model was produced with the commercial program FLUENT. The theoretical model has been developed and validated by comparing the predictions of the model with available experimental data. The thermal performance and temperature distribution for the louvered fins were analyzed and a procedure for optimizing the geometrical design parameter is presented.

One fin specification among the various flat tube exchangers is recommended by first considering the heat transfer and pressure drop. The effects of variation of coolant flow conditions and external air conditions on the flow and the thermal characteristics for the selected radiator are investigated also. The results will be used as fundamental data for tube design by suggesting specifications for car radiator tubes.     

The effects of component dimensions on heat transfer and pressure loss in shell and tube heat exchangers

In this study, the effects of geometric properties such as baffle spacing, baffle cut, sealing strips, gaps between heat exchanger components, number of tubes and tube passes have been investigated for shell and tube heat exchangers (STHE). For this purpose, geometrical dimensions and data of a specific heat exchanger (HE) used in an industrial application have been obtained. The HTRI Xchanger Suite Educational software was utilized to analyze the reference HE, to verify the results of the software and the output temperatures of the fluids, the shell side heat transfer coefficient (HTC) and the pressure loss (PL) values were compared with the experimental data. After confirming that the results of the software were within the acceptable deviation values, the geometric dimensions of the reference design were changed and new analyzes were carried out to examine the effects of several dimension options on the performance of STHE. Results were discussed in a detailed and comprehensive manner via curves.     

Performance Optimization of a Class of Finite Time Heat Engines

The effect of heat resistance and heat leakage on the optimal performance of finite time heat engines is investigated in this paperbased on a generalized heat transfer law q ∞ Δ(Tⁿ). The analytical relation between optimal power output and efficiency for steady-state flow irreversible heat engines is derived. The analysis includes the optimal performance characteristics of several types of heat engines with different loss item and different heat transfer laws. A numerical example is provided for illustrating the power output versus efficiency characteristics. Results shown that the heat transfer law does affect the performance of these heat engines.     

Wintertime Surface Heat Exchange for the Inner Mongolia Reach of the Yellow River

This study analyzed the wintertime surface heat exchange for the Inner Mongolia reach of the Yellow River, China, based on the data from the nearby weather station at Wulateqianqi. In this analysis, the solar radiation is based on the observed data. Other components of the surface heat flux, that is, long‐wave radiation, and evaporative and conductive heat fluxes, are calculated. The relative importance of the contributions of long‐wave radiation, conductive, and evaporative heat fluxes are in descending order. The air temperature is the most important meteorological factor to the total heat flux. Although the wind speed influences evaporative and conductive heat fluxes, it has the least correlation with the total heat budget. The heat exchange coefficient for the linearized surface heat exchange equation is 21.87 W/(m² °C), which is comparable with published values in the regions of United States and Canada with similar latitudes.     

Transport of phosphate through artificial macropores during film and pulse flow

Flow through artificial macropores may occur as a water film along the macropore walls (film flow) or as moving water segments separated by air bubbles (pulse flow). To investigate the effect of macropore flow pattern (i.e., film and pulse flow) on the interaction of solutes with macropore walls, we studied orthophosphate (P) transport and sorption in artificial macropores. The experimental setup consisted of a column (height = 20 cm, diameter = 20 cm) homogenously packed with glass beads and fitted at outflow with a vertical artificial macropore placed below the column. The artificial macropore consisted of ceramic tubes (3 or 8 mm i.d.; 31.5 cm long) coated on the inside with iron oxide serving as phosphate sorbents. An orthophosphate solution containing 0.04 mg P L(-1) was applied at a rate of 9 to 12 mm h(-1) to the column, eventually causing macropore flow. In the 8-mm-i.d. tubes only film flow occurred. Pulse flow was dominating in the 3-mm-i.d. tubes. Generally, the flow patterns were reproducible and seldom did pulse flow replaced film flow or vice versa. During film flow, a significantly larger decrease in macropore P concentration per tube was observed relative to that with pulse flow events. However, pulse and film flow lead to almost the same amounts of P sorbed per unit surface area when exposed to the same solute P concentration. Comparison with P sorption capacity experiments indicated that the sorption rate, rather than the sorption capacity, controls the amount of sorbed P during macropore flow in the studied system.     

Numerical Analysis of Heat Transfer and Gas Flow in PEM Fuel Cell Ducts by a Generalized Extended Darcy Model

Abstract

In this work, gas flow and heat transfer have been numerically investigated and analyzed for both cathode/anode ducts of proton exchange membrane (PEM) fuel cells. The simulation is conducted by solving a set of conservation equations for the whole domain consisting of a porous medium, solid structure, and flow duct. A generalized extended Darcy model is employed to investigate the flow inside the porous layer. This model accounts for the boundary-layer development, shear stress, and microscopic inertial force as well. Effects of inertial coefficient, together with permeability, effective thermal conductivity, and thickness of the porous layer on gas flow and heat transfer are investigated.     

Impact of nonuniform reactant flow rate on the performance of proton exchange membrane fuel cell stacks

ABSTRACT

In this study, a proton exchange membrane (PEM) fuel cell stack composed of five cells in series is numerically investigated to study the impact of the nonuniform reactant flow rate on the performance of the stack. A comparison of the water concentration, temperature, reaction heat source, and current density of change rule of two groups of fuel cell stacks with uniform and nonuniform reactant flow rate reveals the performance degradation mechanism caused by nonuniform reactant flow. The results indicate that while operating under low-voltage conditions, the nonuniform reactant flow rate will cause the accumulation of excess liquid water near the PEM that is near the cathode exhaust outlet, and the local area reacts strongly on the catalyst, whereas the local area reacts slowly. When the average voltage of the stack is 0.55 V, the current density under the nonuniform reactant flow rate condition is 12.9% lower than that of the uniform reactant flow rate condition. In the case of uniform and nonuniform reactant flow rate at low current densities, the performance difference is not evident, but it is expected to be pronounced with the increase in current density. The simulation results are compared with the experimental data reported in the literature through a polarization curve, and they turn out to be well correlated with the experimental results.