Transpiration cooling systems

The evaluation of the effectiveness of a transpiration cooling system utilizing a foreign gas is quite formidable and requires the use of complex computer codes. 

Examples of thermal protection offered by an upstream transpiration system are indicated in Fig. 6.17 (see Ref. 37). The figure shows the temperature rise that occurs on an insulated surface downstream of a transpiration-cooled section for two amounts of blowing. Also indicated are the corresponding surface temperatures for the case where the upstream section was cooled internally, i.e., /(0) = 0. The quantity TwL is the upstream surface temperature in either case. The differences between the curves labeled /(0) = -0.5 and /(0) = -1 and the /(0) curve show how the presence of the transpired coolant within the downstream boundary layer distorts the temperature profiles so as to afford greater downstream protection depending on the amount of blowing. 

Two concentric porous spherical shells (radii R and R2) are used in a cooling system. Air is charged to the inner sphere where it is cooled and then moves through the inner shell to the outer shell and then to the atmosphere (i.e. transpires). The inner surface of the outer sphere is at T2 and the outer surface of the inner sphere is at a lower temperature Tq. Relate heat removal to mass flow rate of the gas. 

Transpiration is the movement of water from the root system up to the leaves and its subsequent evaporation to the atmosphere. This process moves nutrients throughout the plant and cools the plant. Respiration is a heat-producing process resulting from the oxidation of carbohydrates by O2 to form CO2 and H2O, as shown in Eq. (8-2). 

A) Agricultural uses. Water is a basic component of all plants and is taken up from the soil via the root system, flowing up the plant by the osmotic gradient between the soil and the air. The water consumption of a crop can be broken down into three parts (1) Constituent water, which is retained as a constituent part of the plant matter and used in combination with carbon dioxide to produce carbohydrates (photosynthesis), and to assist the uptake and transport of nutrients from the soil. (2) Transpiration water, which is taken up by the plant and lost as water vapor through the process of transpiration to provide cooling for aerial structures. (3) Evaporation water, which is lost by evaporation from the surface of the plant. 

---