Rt: dimensionless thermal resistance A reversed trapezoidal fin with fluid in the inside wall is analyzed using a two-dimensional analytical method. Thermal resistance for the reversed trapezoidal fin is presented as a function of the fin length and fin base height. The relationship between the inside fluid convection characteristic number and convection characteristic number over the fin for the same amount of heat loss is analyzed. One of the results shows that the thermal resistance decreases parabolically with the increase of fin base height. s: fin lateral surface slope, {(1- ξ )lh}/(le-lb) T: fin temperature [℃] Tb: fin base temperature [℃] Tf: inside fluid temperature [℃] T∞: ambient temperature [℃] x: length directional variable [m] X: dimensionless length directional variable, x/lc Nomenclature y: height directional variable [m] Y: dimensionless height directional variable, y/lc Greek symbolh: heat transfer coefficient over the fin [W/m2 ℃] fϕ : adjusted inside ...