tailieunhanh - Heat Transfer Handbook part 29

Heat Transfer Handbook part 29. The Heat Transfer Handbook provides succinct hard data, formulas, and specifications for the critical aspects of heat transfer, offering a reliable, hands-on resource for solving day-to-day issues across a variety of applications. | DEFINITIONS OF SPREADING AND CONSTRICTION RESISTANCES 271 Heat Source Figure Heat flow lines and isotherms for steady conduction from a finite heat source into a half-space. From Yovanovich and Antonetti 1988. doubly connected areas . circular or annular area . The free surface of the halfspace is adiabatic except for the source area. If heat enters the half-space the flux lines spread apart as the heat is conducted away from the small source area Fig. then the thermal resistance is called spreading resistance. If the heat leaves the half-spaee throggh a small araa the flxx lines are constricted and the thermal resistance is called constriction resistance. The heat transfer may be steady or transient. The temperature field T in the half-space is in general threedimensional and steady or transient. The temperature in the source area may be twodimensional and steady or transient. If heat transfer is into the half-space the spreading resistance is defined as Carslaw and Jaeger 1959 Yovanovich 1976c Madhusudana 1996 Yovanovich and Antonetti 1988 Rs q Tsink K W where Tsource is the source temperature and Tsink is a convenient thermal sink temperature and where Q is the steady or transient heat transfer rate Q II qn dA jj -kd- dA W where qn is the heat flux component normal to the area and dT dn is the temperature gradient normal to the area. Ifthe heat flux distribution is uniof rm over the area Q qA. For singly and doubly connected source areas three source temperatures have been used in the definition maximum temperature centroid temperature and area-averaged temperature which is defined according to Yovanovich 1976c as T source source dA K A 272 THERMAL SPREADING AND CONTACT RESISTANCES where A is the source area. Because the sink area is much larger than the source area it is by convention assumed to be isothermal . Tsink Tœ . The maximum and centroid temperatures are identical for singly connected axisymmetric source areas otherwise .