Air-water Heat Exchanger Research Articles

Stream Temperature Dynamics in Upland Agricultural Watersheds

A numerical model to compute the free-surface flow hydrodynamics and stream temperature dy- namics by solving the depth-averaged, 1D unsteady flow and heat transport equations is presented. The hydro- dynamics model considers the effects of arbitrary stream geometry, variable slopes, variable flow regimes, and unsteady boundary conditions. The thermal transport model accounts for the effects of solar radiation, air tem- perature, relative humidity, cloud cover, wind speed, heat conduction between water and streambed, subsurface flow, and shading by riparian vegetation. The model is verified with measurements in a stream in an upland agricultural watershed located in Indiana. Diurnal variations in the streamflow and stream temperatures are highly transient. The proposed model predicted well the streamflow and stream temperatures that were measured every 15 min over 25 days. The results of this study demonstrate that the solar (shortwave) radiation and subsurface inflow are the most significant contributors to the stream heat budget. model was successfully applied in a steep channel where tem- poral change in flow is not abrupt. The solution of the com- plete hydrodynamics equation is necessary to incorporate the inertial terms, variety of channel slopes, and abrupt boundary conditions in small streams. In this paper, fully hydrodynamic and heat transport models are developed and coupled for water temperature and stream- flow prediction in streams. The model considers air-water heat exchange, sediment-water heat exchange, lateral heat inflow/ outflow, subsurface inflow/outflow, and the interaction be- tween solar radiation and riparian vegetation. The proposed model is verified with measurements in Little Pine Creek, Tip- pecanoe County, Ind. Good agreement between field measure- ments and model prediction is reported.

Study of atmospheric-turbulence effects on the formation of a thermal film in the near-surface water layer and the dynamics of air-water heat exchange using measurements of thermal radio emission

On the basis of radiometric measurements of thermal radio emission from water at wavelengths 2.3 and 5 mm, we study the dynamics of heat and mass transfer through a water-air boundary, which is related to atmospheric turbulence, and reconstruct the behavior of the water-temperature profile and the heat flux through this boundary. The heat-flux components due to evaporation and thermal conductivity and the evaporation rate from unit surface are determined. Turbulence-induced variations in the depth of the atmospheric viscous sublayer are studied. Statistical parameters of the water-surface temperature variations are calculated. We find that these variations were much greater than the turbulent fluctuations of the air temperature under the measurement conditions.

Air temperature prediction model to control solar energy heating in a germination greenhouse at night

Summary. The outdoor air temperature is not constant especially in spring in Okayama city. The average night temperature ranges from –2 to 20°C which is too low for the germination of most seeds. A good knowledge of the future outdoor air temperature is necessary to decide if greenhouse heating is needed for the next day. Using measured temperatures from the preceding days and considering the minimum and maximum temperatures given by the weather forecast, it was possible to accurately compute the temperature for the next day. Pumpkin, eggplant and tomato seeds were used in this study. A solar system was used to heat the air inside a greenhouse at night using an air–water–air heat exchanger and make a comparison with an unheated greenhouse. The performance of the solar collector and methods of heat exchange were tested. It was shown that the solar energy collected was sufficient for warming a nursery greenhouse overnight. The system operated with a hot water flow rate of 0.647 L/min and an air flow rate of 9.21 m3/min and could maintain the greenhouse temperature between 16 and 20°C. The quantity of heat collected and delivered by the solar system from incident solar radiation was about 50% in a day. Heating the air inside the greenhouse at night produced 100% germination for all seedlings. In comparison, in the unheated greenhouse the germination ratio was 100, 93 and 27% for pumpkin, eggplant and tomato respectively. The germination ratio outside the greenhouses was 100% for pumpkin, 67% for eggplant and zero for tomato.

自然光利用型植物工場の夏期昼間の局所冷房 トマトの尻腐れとの関連も含めて

In this study, air cooling in the zone only around plants in greenhouse in the daytime in summer was investigated.Two types of air cooling systems were constructed in a greenhouse in Ehime University. One was the system which used cold water and water-air heat exchanger (cold water cooling system), and the other was the system which utilized heat of vaporization of fog (fog cooling system). Schematic diagrams of these systems are shown in Fig. 1 and Fig. 2.The results of air cooling on a fine day are shown in Fig. 4, Fig. 5 and Fig. 6. Furthermore, Fig. 7 shows the effectiveness of system identification in analysis and control in cold water cooling system.The effect of environment of above-ground part on occurrence of blossom-end rot of tomato plants was also investigated. The effect of air flow to fruits was also examined using the system shown in Fig. 3. The results are shown in Fig. 8 and Table 1.

Water Thermal Storage Type Solar Greenhouse

The thermal performance of a solar heating greenhouse designed for a hydroponic system was studied. The system was constructed with an air-water heat exchanger and heat storage tanks that were combined with hydroponic water beds.The heat exchanger had been designed for a moderate heat source. To achieve a reasonable price and a compact heat exchanger, the heat transfer surface is made of a polyethylene film shaped into a bag; many poly-bags are installed into a casing in parallel and close to each other. The heat transfer is accomplished as follows: Water flows down inside the bags in a thin layer, and air flows outside the bags.The water tanks to store solar heat were separated into two parts with a polyethylene film. The upper parts were filled with hydroponic water used for the plant growth bed, and the lower parts were filled with fresh water. The fresh water was circulated into the heat exchanger to recover the solar heat inside the greenhouse and to release the heat for heating.The system was designed in 3.6m×9.0m greenhouse equipped with an interior curtain. The total heat transfer surface in the heat exchanger was 28m2 and the heat transfer rate was 16kcal/m2h°C. The two water tanks (6m long, 0.6m wide and 0.5m deep) were installed and filled with 0.8m3 of hydroponic water and 2.8m3 of fresh water. The tanks were buried 0.35m deep under the ground to keep favorable working conditions. A 2.08kW electric water heater was installed into the tank for auxiliary heating.Cucumbers were grown and the data were collected through seventy days from Jan. 5 in 1983.The averaged minimum air temperature inside the greenhouse was maintained at 11.4°C, the solar heat storage efficiency was 0.18 and solar heat provided 77% of the total heat for heating through seventy days.The solar heat storage efficiency depended on outside insolation. When outside insolation was less than 1000kcal/m2 day, the solar heat storage efficiency was zero, and was 0.32 when outside insolation was over 3000kcal/m2 day. The highest COP of the system (the heat released from the heat storage tank/the energy consumption to store and release the solar heat) was 6.6 and the averaged COP was 4.1. The COP was also in proportion to outside insolation. In the days when the COP was over 4.1, the heating was achieved with only the solar heat storage.The hydroponic water temperature was maintained at 24-15°C by the heat from the fresh water, high enough to grow cucumbers. Therefore, it was not necessary to apply another heat source to heat the hydroponic water.The cucumbers grew well and the system appeared to be able to supply 77% of heating energy from solar heat. It would be necessary for further research to optimize carbon dioxide and humidity levels without deteriorating the capability of solar heating for practical use.

Construction of a practical and inexpensive air stimulator for caloric vestibular testing.

An inexpensive and practical method for delivering air into the aural canal at a selected stable temperature is described. A water-air heat exchanger utilizing the external circulation of a constant-temperature water bath brings the temperature of the air stream to a chosen value. Measurements indicate stable air outflow temperatures are maintained when proper nozzle design and air flow rates are employed. The apparatus is easy to construct and has proven convenient and effective when used in the clinical vestibular laboratory.

Dynamics of a cross-flow heat exchanger with fins

Considerable temperature gradients are often found in the stationary case in the single fins of an air-water heat exchanger. A theoretical investigation of the effect of the dynamics of the fins has been performed for the over-all heat exchanger dynamics. A model which includes radial and axial temperature profiles is compared with a simple model where radial gradients are neglected. For most practical cases it is found that the simple model represents the dynamics very well at frequencies below 10–15 rad/min. The model for the fin is solved with high accuracy using a low order orthogonal collocation approach and the complicated 3-dimensional heat exchanger model is thus reduced to a computationally simple system.

Concurrent Reduction of Draft Height and Heat-Exchange Area for Large Dry Cooling Towers

A procedure is outlined to meet simultaneous requirements to reduce overall size of a dry cooling tower for a large power plant, and to reduce the size (surface area) of the associated air-water heat exchanger. First, tower exit dimensions (or fan power) are specified as attainable fractions of their theoretical minima as found from a draft equation. Then a heat-exchanger type is chosen, having as small an air hydraulic diameter as feasible. Appropriate equations and assumptions dealing with air side and water side heat exchange and water pumping power then yield a full description of tower and heat-exchanger characteristics for a given tower duty. A specific example is worked out and compared with the tower at Rugeley, England. We find that a very open heat exchanger, of shallow depth (one in or less) results from our analysis, and in a proposed configuration of acceptable header loss gives a 1/3 height reduction and a four-fold reduction of heat-exchanger area.