Convection

Pagina 1

1 HEAT TRANSFER & THERMAL ANALYSIS Convection – 1 Problem 1 Air at P = 6 kPa and T∞= 300 °C flows at u∞ = 10 m s-1 over a flat plate long L = 0.5 m. Calculate thermal power per unit width required to maintain the flat plane at Tw = 27 °C. Air properties: cinematic viscosity ; thermal conductivity ; Pr = 0.687. Solution Gas viscosity does not depend on pressure when P<10 bar. By calculating Reynolds number, we can determine whether the flow is laminar or turbulent Flow is laminar. Nusselt number can be calculated by using an adequate correlation for external flow and Pr>0.6 The heat transfer coefficient is Thermal flux can be obtained as Thermal power per unit width to be subtracted is 2 Problem 2 Velocity of air at is measured through the temperaturae of a thin metallic plate 2 cm long, parallel to flow, which is heated on both sides by a total thermal power of . A temperature sensor on the far edge of the plate indicates . Calculate air speed . Air properties at 25 °C: Solution Heat flow is imposed on both sides of the plate. The temperature of the plate depends on the distance x. Air properties have to be calculated at the average temperature . The heat coefficient at the edge of the surface can be computed as To evaluate air speed, it is necessary to find Nusselt number, then the adequate correlation from which Reynolds number might be determined From the correlation of the table for external laminar flow, local value, Pr>0.6, we get 3 Problem 3 In a solar energy system a tube (D=60 mm) is placed in the focus of a parabolic reflector. The effect can be approximated by uniform heating of the tube surface. Consider then a constant thermal flux along the axis of the tube. Water flow rate is ; inlet water temperature . (a) What is the length L required so that outlet…