Jingchun Min

Condensate formation and drainage on typical fin materials

This work addresses contact angles on aluminum and copper surfaces and viable cleaning methods to promote surface wetting. Although the work involved fin materials used in dehumidifying heat exchangers, it has broader application, such as copper-water heat pipes. Condensate visualization tests were conducted on four typical fin surface materials, including aluminum, copper, and two commercial coatings on aluminum. Additional samples were created by further treatment of the basic surfaces. The treatments included acetone cleaning, grinding, and oil contamination. Observations were made using an apparatus that allows viewing of the condensate formed on a vertical flat surface with a humid air stream moving over it. The observations were quantified by measuring the condensate retention and the receding contact angle. Surface grinding yielded small receding contact angle of the aluminum and copper stocks; but, acetone cleaning was not effective for the bare aluminum and copper fin stocks. The surfaces having small receding contact angle provided film sheeting. However, condensate formed as droplets on those having higher receding contact angles. The weight of condensate retained per unit area was found to be an approximate function of the receding contact angle. As the receding contact angle increases, the retained condensate increases, reaching a maximum at approximately 40°, and then begins to decrease.

Long-Term Hydraulic Performance of Dehumidifying Heat-Exchangers With and Without Hydrophilic Coatings

An experimental study has been carried out to investigate the long-term wetting characteristics of dehumidifying finned tube heat-exchangers (coils). The coils were subjected to up to 1000 cycles in which they were alternately dipped into distilled water for 15 minutes and then dried with a fan for 45 minutes. Wetting durability was determined for three uncoated coil, two were coated with proprietary coatings, and a coil with a zinc coating sprayed on the downstream face, and with different fin press oils. Wind tunnel tests on fully wet coils under dehumidification were conducted. The advancing and receding contact angles werer measured by periodically removing small fin samples from the coils. One coating maintained the receding contact angle below 15° throughout the cycle tests. The advancing contact angles were much higher than the receding contact angle. The best coating maintained the wet/dry pressure drop ratio below 1.34 throughout the 1000 cycles. The wet/dry pressure drop ratio was correlated as a function of receding contact angle. An effect of oil was observed on uncoated coils in the initial 100 cycles but vanished after 500 cycles.

Long-term wetting and corrosion characteristics of hot water treated aluminum and copper fin stocks

Experimental studies were conducted to investigate the long-term wetting and corrosion characteristics of hot water treated aluminum and copper fin stocks. This was done by subjecting them to wet/dry cycling consisting of alternately dipping the fin samples into distilled water for 5 min followed by air drying with a fan for 25 min. To simulate the fins of a heat exchanger, fin stocks were formed in a wavy shape using hand operated fin dies and a typical lubricant oil. The fin samples, including the formed and the unformed aluminum and copper stocks, were treated using hot water by immersing them in either 82 or 100 °C water for 20 min. The wettability and corrosion evaluations consisted of periodic measurements of the advancing/receding contact angles and weights of the fin samples. SEM photos were taken to show the appearance of the sample surfaces. Further, X-ray photoelectron spectroscope examinations were performed to analyze the atomic composition at the sample surfaces. The test results suggest that the hot water soak is an effective means to improve the surface wettability of both aluminum and copper surfaces. The soaking procedure was capable of producing wettable fin that was durable to wet/dry cycling. The soaking operation and follow-on wet/dry cycling treatment did not cause significant material loss by general corrosion.

Condensate carryover phenomena in dehumidifying, finned-tube heat exchangers

An experimental study has been conducted to investigate the condensate carryover phenomena in dehumidifying heat exchangers. Two wavy finned-tube coils were tested, for which the fin surfaces were treated to provide either low or high contact angles. The receding contact angle on the fins of the two coils were 70° and 10°, respectively. The distribution of condensate carryover was measured along the tunnel bottom downstream from the coil for different air frontal velocities. As the frontal velocity increases, the quantity of condensate carryover increases, and the condensate is blown further from the coil. The receding contact angle on the fin surface is a key factor controlling the condensate carryover characteristics. The coil having a 10° receding contact angle shows significantly less condensate carryover than the coil having a 70° receding contact angle. Numerous condensate droplets and bridges were observed on the fin surfaces of the 70° receding contact angle coil; however, few were seen for the 10° receding contact angle coil. The dominant carryover results from droplets formed from bridged condensate, and the diameter of the resulting droplets is approximately 3.0 mm.

EXPERIMENTAL AND THEORETICAL INVESTIGATIONS OF MEMBRANE-BASED ENERGY RECOVERY VENTILATOR PERFORMANCE

Experimental and theoretical studies were conducted to investigate the performance of a membrane-based energy recovery ventilator, which is an air-to-air heat exchanger with a water vapor permeable core. Tests were done on a commercially available membrane-based energy recovery ventilator, during which the supply and exhaust air states were recorded continuously over a long period of time. The test results show that as time passes the sensible effectiveness decreases very slightly from 56% to 55%, while the latent effectiveness increases appreciably from 28% to 39%. As a resultant of sensible and latent effectiveness, the enthalpy effectiveness provides an intermediate value between them and increases from 39% to 43%. The reason that the effectiveness changes with time is that the air relative humidity changes with time, which alters the moisture content at the membrane surface. Calculations were then implemented to predict the performance of the membrane-based energy recovery ventilator based on the model we developed previously and the calculation results were compared with the experimental data. The comparison suggests that the calculated effectiveness agrees well with the measured one, supporting the reasonability of the model. Calculations were also made to investigate the effects of various membrane parameters on latent and enthalpy effectiveness and latent-to-sensible heat ratio.

An Improved Method for Evaluating the Moisture Diffusivity in a Membrane

A method that combines the experimental measurements and numerical simulations to determine the moisture diffusivity in a membrane has been developed. An experimental set-up was designed and constructed to measure the total moisture resistance. The test section consists of an airflow channel, a membrane, and a water tank, which form a sandwich structure. The process of moisture transport from the water surface to the airstream in the channel is numerically simulated to obtain the variation of the total moisture resistance with the moisture diffusivity in the membrane, which is then determined by comparing the experimental and numerical total moisture resistances. There are three features with the present method, i.e., simple structure of the test section, combination of the experiment and simulation, and consideration of the boundary layer resistances on both sides of the membrane. Tests were conducted on two PVDF membranes with 0.22 and 0.45 μm mean pore diameters. The results show that the moisture diffusivities in both membranes are in the order of 10-6 kgm-1s-1, with a larger pore size tending to yield a larger diffusivity. The moisture diffusivities in both membranes are insensitive to the airflow rate.

Heat and Mass Transfers and their Mutual Effects in Membrane Processes

A mathematical model was developed to describe the coupled heat and mass transfers in membrane processes. Equations for the heat and mass transfer resistances were derived and the coupling effects of the heat and mass transfer were analyzed. With taking the membrane separation process of moist air as an example, the effects of air temperature and water vapor concentration on the heat and moisture transfer process were investigated. The results show that neither the thermal resistance nor the moisture resistance are constant, they are affected by not only the membrane parameters but also the air state. As the temperature difference between the two airstreams separated by the membrane increases, both the thermal and moisture resistances decrease, causing an improved heat and mass transfer. As the average temperature of the two airstreams increases, the thermal resistance remains almost constant while the moisture resistance decreases significantly. Further, as the water vapor concentration difference between the two airstreams increases, both the thermal and moisture resistances increase. As the average water vapor concentration of the two airstreams increases, the thermal resistance remains unchanged while the moisture resistance decreases.

Cooling of laser slab by forced convection through a heat sink

The present study addresses a novel cooling scheme for the high-power solid-state laser slab. The scheme cools the laser slab by forced convection in a narrow channel through a heat sink. Numerical simulations were conducted to investigate the thermal effects of a Nd:YAG laser slab for heat sinks of different materials, including the undoped YAG, sapphire, and diamond. The results show that the convective heat transfer coefficient is non-uniform along the fluid flow direction due to the thermal entrance effect, causing a non-uniform temperature distribution in the slab. The heat sink lying between the coolant fluid and the pumped surface of the slab works to alleviate this non-uniformity and consequently improve the thermal stress distribution and reduce the maximum thermal stress of the slab. The diamond heat sink was found to be effective in reducing both the highest temperature and the maximum thermal stress; the sapphire heat sink was able to reduce the maximum thermal stress but not as effective in reducing the highest temperature; and the undoped YAG heat sink reduced the maximum thermal stress but tended to increase the highest temperature. Therefore, cooling with the diamond heat sink is most effective, and that with the sapphire heat sink follows; cooling with the undoped YAG heat sink may not apply if the highest temperature is a concern.