To cool electronic components that are mounted to a printed circuit board and hermetically sealed from their surroundings, two boards may be joined to form an intermediate channel through which the coolant is passed. Termed a hollow-core PCB, all of the heat generated by the components may be assumed to be transferred to the coolant. Consider a hollow-core PCB of length L = 300 mm and equivalent width W (normal to the page). Under normal operating conditions, 40 W of power are dissipated on each side of the PCB and a uniform distribution of the attendant heat transfer may be assumed for each of the hollow-core surfaces. If air enters a core of height H = 4 mm at a temperature of Tm, i = 20