Closed-loop oscillating heat-pipe with check valves (CLOHP/CVs) air-preheater for reducing relative humidity in drying systems

A CLOHP/CV air-preheater has been used for recovering the waste heat from the drying cycle. The CLOHP/CV heat-exchanger consisted of copper tubes 3.58 m long and internal diameter 0.002 m. The evaporator and condenser sections were 0.19 m long, the adiabatic sections 0.08 m long, the hot air velocity was 0.5, 0.75 or 1.0 m/s with the hot air temperature 50, 60 or 70 °C, and the relative humidity was 100%. The working fluid was R134a with a filling ratio of 50%. The hot-air temperature increased from 50 to 70 °C; the heat-transfer rate increased slightly. The velocity increase from 0.5 0.75, to 1.0 m/s led to the heat-transfer rate slightly decreasing. The velocity increase from 0.5 to 1 m/s led to a slight decrease in effectiveness. As the hot-air temperature increases from 50 to 70 °C, the effectiveness slightly increased; and the relative humidity was reduced to the range 54–72% from 89% to 100%. The CLOHP/CV air-preheater can reduce the relative humidity and achieve energy thrift.     

Application of closed-loop oscillating heat-pipe with check valves (CLOHP/CV) air-preheater for reduced relative-humidity in drying systems

This paper aims to design, construct, and test the CLOHP/CV air-preheater for reduced relative humidity in drying systems for recovering the waste heat from the drying cycle. The CLOHP/CV heat-exchanger consisted of 3.58 m long copper tubes, with an internal diameter of 0.002 m. The evaporator and condenser sections were 0.19 m long, the adiabatic sections were 0.08 m long, the hot-air velocities were 0.5, 0.75 and 1 m/s with the hot-air temperatures being 50, 60 and 70 °C, and the relative humidity was 100%. The working fluid was R134a with the filling ratio of 50%. It can be concluded that, with an increase in the hot-air temperature from 50 to 70 °C, the heat-transfer rate slightly increases. The velocity increases from 0.5, 0.75 to 1 m/s and the heat-transfer rate slightly decreases. The velocity increases from 0.5, 0.75 to 1 m/s and the effectiveness slightly decreases. The hot-air temperature increases from 50 to 70 °C and the effectiveness slightly increases. The relative humidity reduced to the range 54–72% from 89% to 100%. The CLOHP/CV air-preheater can reduce the relative humidity and achieve energy thrift.     

Dimensionless correlations of frost properties on a cold cylinder surface

We propose dimensionless correlations for frost properties on a cold cylinder surface. Frosting experiments were performed while changing various frosting parameters such as the air temperature, cold cylinder surface temperature, air velocity, and absolute humidity. The experimental data showed that a uniform frost layer grew around the circumference of the cylinder at a high air velocity. Dimensionless correlations for the thickness, density, and surface temperature of the frost layer, and for the heat transfer coefficient were obtained as functions of the Reynolds number, Fourier number, absolute humidity, and dimensionless temperature. The applicable ranges of these correlations are Reynolds number of 700–3000 (air velocities of 0.5–2.0 m/s), Fourier number of 56.8–295.7 (operating time of 0–100 min), absolute humidity of 0.00280–0.00568 kg/kg_a, air temperatures of 3–9 °C, and cold cylinder surface temperatures of ?32 to ?20 °C. The proposed correlations agreed with the experimental data within an error of 15%.     

Free convection frost growth in a narrow vertical channel

Processes involving heat transfer from a humid air stream to a cold plate, with simultaneous deposition of frost, are of great importance in a variety of refrigeration equipment. In this work, frost growth on a vertical plate in free convection has been experimentally investigated. The cold plate (0.095 m high, 0.282 m wide) was placed in a narrow (2.395 m high, 0.01 m deep) vertical channel open at the top and bottom in order to permit the natural circulation of ambient air. The cold plate temperature and the air relative humidity were varied in the −40 to −4 °C and 31–85% range, respectively, with the air temperature held fixed at 27 °C (±1 °C). The main quantities (thickness, temperature and mass of frost, heat flux at the cold plate), measured during the time evolution of the process, are presented as functions of the input parameters (relative humidity and cold plate temperature); in particular, the role exerted by the plate confinement on the frost growth is discussed. Data are recast in order to identify compact parameters able to correlate with good accuracy frost thickness, mass and density data.     

A mathematical model for predicting the performance of a compound desiccant wheel (A model of compound desiccant wheel)

A mathematical model for predicting the performance of novel silica gel haloid compound desiccant wheel is established. Both the gas side resistance and the solid side resistance are considered in the model. It is found that the results of this model agree better to the experiments than the results of a former model which does not take the solid side resistance into account at all. Then the model is adopted to analyze the effects of some main parameters on system performance. It is found that the compound desiccant wheel has a better performance in a climate with moderate temperature or in a climate with high humidity ratio. Then under the basic conditions for the simulations (ambient air is of 35 °C, 15 g/kg and wheel thickness of 100 mm), an angle of the regeneration section between 100° and 160°, a regeneration temperature between 80 °C and 95 °C, a flow rate of process air between 2.0 m/s and 3.5 m/s and that of regeneration air between 2.5 m/s and 3.5 m/s are recommended. Also, there exists an optimal rotation speed to achieve the maximal moisture removal, which is about 12 r/h. At last, the influences of the main parameters on optimal rotation speed are discussed.     

Performance assessment of a direct expansion air conditioner working with R407C as an R22 alternative

This paper presents an experimental investigation of a direct expansion air conditioner working with R407C as an R22 alternative. Experiments are conducted on a vapor compression refrigeration system using air as a secondary fluid through both the evaporator and the condenser. The influences of the evaporator air inlet temperature (20–32 °C), the evaporator air flow rate (250–700 m³/h) and the evaporator air humidity ratio (9 and 14.5 g_wv/kg_a) at the condenser air temperature and volume flow rate of 35 °C and 850 m³/h, respectively on the system performance are investigated. Experimental results revealed that the evaporator air inlet temperature has pronounced effects on the air exit temperatures, pressures of the evaporator and the condenser, cooling capacity, condenser heat load, compressor pressure ratio and the COP of both refrigerants at humidity ratios of 9 and 14.5 g_wv/kg_a. Significant effects of the evaporator air flow rate are also gathered on the preceding parameters at the same values of mentioned-humidity ratios. The best performance, in terms of operating parameters as well as COP, can be accomplished using R22 compared to R407C. The inlet humidity ratio affects dramatically the performance of vapor compression system using R22 and R407C. The raising up humidity ratio from 9 to 14.5 g_wv/kg_a leads to an augmentation in the average cooling capacity by 29.4% and 38.5% and an enhancement in the average COP by 30% and 24.1% for R22 and R407C, 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.     

The role of ambient conditions on the performance of a planar,air-breathing hydrogen PEM fuel cell

This paper presents experimental data on the effects of varying ambient temperature (10–40 °C) and relative humidity (20–80%) on the operation of a free-breathing fuel cell operated on dry-hydrogen in dead ended mode. We visualize the natural convection plume around the cathode using shadowgraphy, measure the gas diffusion layer (GDL) surface temperature and accumulation of water at the cathode, as well as obtain polarization curves and impedance spectra. The average free-convection air speed was 9.1 cm s⁻¹ and 11.2 cm s⁻¹ in horizontal and vertical cell orientations, respectively. We identified three regions of operation characterized by increasing current density: partial membrane hydration, full membrane hydration with GDL flooding, and membrane dry-out. The membrane transitions from the fully hydrated state to a dry out regime at a GDL temperature of approximately 60 °C, irrespective of the ambient temperature and humidity conditions. The cell exhibits strong hysteresis and the dry membrane regime cannot be captured by a sweeping polarization scan without complete removal of accumulated water after each measurement point. Maximum power density of 356 mW cm⁻² was measured at an ambient temperature of 20 °C and relative humidity of 40%.     

Transient and steady-state natural convection heat transfer from a heated horizontal concrete cylinder

In refrigeration systems, it is possible to reduce energy consumption (compressor power) and increase COP by decreasing the condensation temperature. Decreasing the condensation temperature can be achieved either by increasing the overall heat transfer coefficient or heat transfer surface area of the condenser. Usually, the radiuses of condenser tubes of domestic refrigerators are quite smaller than the critical radius. Thus, the radius can be increased up to the critical radius by coating the bare condenser tube to increase heat transfer. On the other hand, refrigerators operate discontinuously depending on the ambient temperatures. Coating material stores some of the heat during the working period and continues heat transfer during the off period so that the condenser continues transferring heat while the compressor is not working. Storage effect depends on the specific heat and density of the coating material. Transient and steady-state natural convection heat transfer from a heated horizontal cylinder covered with concrete layer by molding is studied experimentally and numerically to determine the effects of the parameters considered above. The copper and the concrete test cylinders used in the experimental study have a length of 1 m and outer diameter of 9.45 mm and 68.5 mm respectively. The ambient and copper cylinder surface temperatures varied between 20 °C÷30 °C and 30 °C÷50 °C respectively. Constant heat flux was applied to bare and concrete cylinders. Transient heat transfer experiments were performed when bare, and concrete cylinders were reached to steady state condition. Heat transfer rates under transient conditions from bare and concrete horizontal cylinders were compared and heat transfer enhancement was determined. Based on the experimental data average Nusselt numbers were calculated and compared with the well known correlations. Also temperature distributions obtained from numerical simulations were very close to the experimental data. The effect of the decrease in the temperature of the inner copper cylinder surface (condensation temperature) on COP was investigated considering an ideal Carnot refrigeration cycle. It is found that the enhancement in COP of a Carnot refrigeration cycle is 35.7% under transient condition.     

Mathematical modeling of drying characteristics of tropical fruits

In this study, a heat pump dryer was designed and produced in which drying air temperature was controlled with PID. PID controlled heat pump dryer was experimentally tested in drying tropical fruits such as kiwi, avocado and banana. Drying air temperature was kept at 40 °C with the accuracy of ±0.2 °C. Drying air velocity changed between 0.03 m/s and 0.39 m/s. Initial moisture contents of the kiwi, avocado and banana were 4.31, 1.51 and 4.71 g water/g dry matter, which were dried to 0.75, 0.35 and 0.5 g water/g dry matter moisture contents in 6 h. Afterwards the moisture ratios were analyzed with “STATGRAPHIC” computer program by using semi-theoretical models and empirical values. Correlation and standard error of estimation (SEE) and R² values were achieved.     

Operation of PEM fuel cells at 120–150 °C to improve CO tolerance

Sulfonic acid modified perfluorocarbon polymer proton exchange membrane (PEM) fuel cells operated at elevated temperatures (120–150 °C) can greatly alleviate CO poisoning on anode catalysts. However, fuel cells with these PEMs operated at elevated temperature and atmospheric pressure typically experience low relative humidity (RH) and thus have increased membrane and electrode resistance. To operate PEM fuel cells at elevated temperature and high RH, work is needed to pressurize the anode and cathode reactant gases, thereby decreasing the efficiency of the PEM fuel cell system. A liquid-fed hydrocarbon-fuel processor can produce reformed gas at high pressure and high relative humidity without gas compression. If the anode is fed with this high-pressure, high-relative humidity stream, the water in the anode compartment will transport through the membrane and into the ambient pressure cathode structure, decreasing the cell resistance. This work studied the effect of anode pressurization on the cell resistance and performance using an ambient pressure cathode. The results show that high RH from anode pressurization at both 120 and 150 °C can decrease the membrane resistance and therefore increase the cell voltage. A cell running at 150 °C obtains a cell voltage of 0.43 V at 400 mA cm⁻² even with 1% CO in H₂. The results presented here provide a concept for the application of a coupled steam reformer and PEM fuel cell system that can operate at 150 °C with reformate and an atmospheric air cathode.     

Evaluation of convective heat transfer coefficient of various crops in cyclone type dryer

In this paper, an attempt was made to evaluate the convective heat transfer coefficient during drying of various crops and to investigate the influences of drying air velocity and temperature on the convective heat transfer coefficient. Drying was conducted in a convective cyclone type dryer at drying air temperatures of 60, 70 and 80 °C and velocities of 1 and 1.5 m/s using rectangle shaped potato and apple slices (12.5 × 12.5 × 25 mm) and cylindrical shaped pumpkin slices (35 × 5 mm). The temperature changes of the dried crops and the temperature of the drying air were measured during the drying process. It was found that the values of convective heat transfer coefficient varied from crop to crop with a range 30.21406 and 20.65470 W/m² C for the crops studied, and it was observed that the convective heat transfer coefficient increased in large amounts with the increase of the drying air velocity but increased in small amounts with the rise of the drying air temperature.     

Grape drying by using hybrid photovoltaic-thermal (PV/T) greenhouse dryer: An experimental study

A hybrid photovoltaic-thermal (PV/T) greenhouse dryer of 100 kg capacity has been designed and constructed at Solar Energy Park, Indian Institute of Technology, New Delhi (28°35′N, 77°12′E, 216 m above MSL), India. The developed dryer has been used to dry the Thompson seedless grapes (Mutant: Sonaka) when DC fan was in operation for forced mode convection. The drying of grapes was also performed in open as well as shade for comparison. Experiments were conducted for drying of grapes in the month of April, 2007. Various hourly experimental data namely moisture evaporated, grape surface temperatures, ambient air temperature and humidity, greenhouse air temperature and humidity, etc. were recorded to evaluate heat and mass transfer for the proposed system. It has been found that the value of the convective heat transfer coefficient for grapes (GR-I) lies between 0.26 and 0.31 W/m² K for greenhouse and 0.34–0.40 W/m² K for open conditions, respectively and that for grapes (GR-II) lies between 0.45–1.21 W/m² K for greenhouse and 0.46–0.97 W/m² K for open conditions, respectively.     

Design and fabrication of a cost effective solar air heater for Bangladesh

In this research, a solar air heater is designed, fabricated and its performance is assessed in the perspective of an emerging/developing country with a huge energy demand like Bangladesh. The winter season (mid-November–mid-February) of the country characterizes by low temperatures, cool air blowing from the west or northwest, clear sky and meager rainfall. Minimum temperature in the extreme northwest in late December and early January sometimes reaches 3 °C and day length is about 10 h. The shortness of winter days can be compensated by reducing the heat loss during long nights. The solar air heater is constructed to prevent as much heat loss as possible. In other words, the heating of air is accomplished by maximizing light gain and minimizing heat loss. It is observed that the fabricated solar air heater is working efficiently. The maximum room temperature and the temperature difference from ambient are 45.5 °C and 12.25 °C for forced circulation and 41.75 °C and 8.5 °C for natural circulation respectively. The experimental outlet temperatures have been compared with that of theoretical values. Due to its low-cost and simple technology, it is affordable in all aspects, viz. of cost, operation and maintenance by the typical people of Bangladesh.     

Heat transfer during condensation of moving steam in a narrow channel

The paper presents the results of experimental investigation of heat transfer and hydrodynamics during condensation of moving steam in a narrow channel of square cross-section 2 mm × 2 mm. The channel had a serpentine shape, the channel length was 660 mm. An experimental cell simulated conditions of heat transfer in the condenser of loop heat pipes. The steam velocity at the channel inlet ranged from 13 to 52 m/s, the pressure was 1 atm. The temperature of the cooling water varied from 70 to 95 °C. The annular flow pattern was noted in the whole range of the regime parameters. There was a clear boundary between the condensation zone and the zone occupied by the condensed phase downstream. Temperature has measured along the channel, and the heat-transfer coefficients have been determined. The coefficient values varied from 10,000 to 55,000 W/K m² depending on the steam velocity at the channel inlet and the cooling temperature. The efficiency of the condenser – heat exchanger has been investigated.     

Performance of a proposed complete wetting surface counter flow channel type liquid desiccant air dehumidifier

An idea that improves the wettability over the surfaces of a cylindrical dehumidifier channel was proposed and experimentally proved. Fibrous sheets were attached to the inner surfaces of the channel. The capillary effect of fibers sustains the complete wetting of the heat and mass transfer surfaces. The air to be dehumidified and cooled flows upward in the annulus space between the two layers of fibrous sheets, which are saturated with the downward flowing desiccant solution. The permeability of the fibrous sheet was determined experimentally. It was 2.43 × 10^?10 m². The measured solution flow rate due to the capillary suction of the sheets was Γ_in,min = 1.12 kg/h m. The liquid desiccant tested was H₂O/CaCl₂ with salt concentration ratios ranging from 35 to 40%. The measured distribution of the solution flow rate along the circumference of the sheets at the outlet showed 5% deviation from the average flow rate. This is a good indication for the good wettability of walls inside the dehumidifier.Feeding the solution by this mechanism has many advantages over spray feeding. Beside sustaining complete surface wetting, it also prevents channel blockage with solution, which is a main factor in increasing the air pressure drop. About 95% of the air pressure drop is saved in this study by avoiding these problems. A simple theoretical model for the heat and mass transfer processes inside the dehumidifier was developed and experimentally validated. In general, there is good agreement between the predicted and measured data. The developed model was utilized to study the effect of the different parameters on the dehumidifier performance. For a 1 m height dehumidifier with an inlet specific humidity and air temperature of 0.0234 kg_v/kg_a, and 35 °C, respectively, the predicted outlet air specific humidity was 0.0102 kg_v/kg_k and the corresponding outlet air temperature was 27.4 °C. The inlet solution temperature and salt concentration were 25 °C and 40%, respectively.     

Experimental results on operating parameters influence for an adsorption refrigerator

A continuous heat recovery adsorption refrigerator using activated carbon-methanol has been developed. In this system, the heat source to drive the adsorption system can be controlled at a temperature from 60 °C to 110 °C, and the evaporating temperature can also be controlled at any requested value from 0 °C to 15 °C. To realize the operation performance of the system, many sensors of temperature, pressure and flow rate are installed in the adsorbers, the condenser and the evaporator. A lot of experiments have been completed in different operation conditions. Thus, by means of the experimental data, influences of the operating parameters, such as heat source temperature, evaporating temperature, cooling water temperature, cycle time and flow rate of throttling valve and so on, on p-t-x diagram of the cycle, specific cooling power (SCP) and coefficient of performance (COP) have been asserted. And causes of the influence are also analyzed. A series of conclusions are obtained.     

A fast response heat pump water heater using thermostat made from shape memory alloy

The heat pump water heater produces hot water so slow at low ambient temperature that it frequently could not meet the hot water load demand in winter. The present study develops a fast response heat pump water heater (FRHP) designed with two separate tanks (supply and holding tank) which are connected by a thermostat made from shape memory alloy (SMAV). The SMAV is a mechanical heat-sensitive device made from shape memory alloy which keeps the valve closed when the water temperature is not high enough. This will isolate the tanks and let the vapor compression cycle heat up the supply tank only. The speed of temperature rise thus is increased. The SMAV will open and induce a natural circulation between two tanks to transfer the heat from the supply tank to the holding tank, when water is heated to a designated temperature. A 100 l FRHP was built and tested in the present study. The experimental results showed that the temperature response speed of the supply tank, before SMAV is opened, reaches 1.056 °C/min and the holding tank, after SMAV is opened, reaches 0.828 °C/min at ambient temperature 20 °C. The FRHP will heat up 50 l water in the supply tank with 30 °C temperature rise within 40 min in winter which is acceptable in domestic application. The energy consumption is in the range 0.008–0.016 kWh/l of hot water at about 55 °C.     

Athermalization of infra-red camera of projectile weapons

One of the vital components of projectile weapons is Infra-Red Camera (IRC) which detects target by virtue of Infra-Red imaging. The lenses of IRC are designed to operate at a temperature of (+)30 °C ± 1 °C. When these weapons were fired from Siachen glacier region, where temperature is around (?)20 °C, they posed the problem of missing their targets. Similar problem was encountered while firing from Thar desert region, where temperature is around (+)60 °C. Hence, the optical lens of IRC is required to be brought from the prevailing ambient temperature of (+)60 °C or (?)20 °C to (+)30 °C ± 1 °C within a very short span, say 60 s to render it suitable for military applications.The objective of this paper is to outline novel mechanism by which uniform temperature of (+)30 °C ± 1 °C from the ambient temperature of (?)20 °C or (+)60 °C can be achieved within 60 s using thermoelectric heating/cooling devices acting as heat pumps. It was found that two of the lenses made of germanium attained the temperature of (+)30 °C ± 1 °C much before 60 s, while the third lens made of AMTIR-I did not attain the required temperature in stipulated time.     

Air heat exchangers with long heat pipes: Experiments and predictions

This paper presents measurements and predictions of a heat pipe-equipped heat exchanger with two filling ratios of R134a, 19% and 59%. The length of the heat pipe, or rather thermosyphon, is long (1.5 m) as compared to its diameter (16 mm). The airflow rate varied from 0.4 to 2.0 kg/s. The temperatures at the evaporator side of the heat pipe varied from 40 to 70 °C and at the condenser part from 20 to 50 °C. The measured performance of the heat pipe has been compared with predictions of two pool boiling models and two filmwise condensation models. A good agreement is found. This study demonstrates that a heat pipe equipped heat exchanger is a good alternative for air–air exchangers in process conditions when air–water cooling is impossible, typically in warmer countries.