The temp distribution within the Laminar thermal boundary layer over a heated isothermal flat plate is given by
$$\left( {T - {T_w}} \right)/\left( {{T_\infty } - {T_w}} \right) = \left( {3/2} \right)\,\,\left( {y/{\delta _t}} \right) - \left( {1/2} \right){\left( {y/{\delta _t}} \right)^3},$$
where $${{T_w}}$$ and $${{T_ \propto }}$$ are the temp of plate and free stream respectively, and $$'y'$$ is the normal distance measuread from the plate. The ratio of Average to the local Nussult number based on the thermal boundary layer thickness $${{\delta _t}}$$ is given by

The average heat transfer coefficient on a thin hot vertical plate suspended in still air can be determined from observations of the change in plate temperature with time as it cools. Assume the plate temp to be uniform at any instant of time and radiation heat exchange with the surroundings is negligible. The ambient temperature is $${25^ \circ }C,$$ the plate has a total surface area of $$0.1{m^2}$$ and a mass of $$4$$ kg.

The specific heat of the plate material is $$2.5KJ/KgK.$$ The convective heat transfer coefficient in $$W/{m^2}K,$$ at instant when the plate temp is $${225^ \circ }C$$ and the change in plate temp with time $$dT/dt=-0.02K/s,$$ is

An un-insulated air conditioning duct of rectangular cross section $$1\,\,m \times 0.5\,m,$$ carrying air at $${20^ \circ }C$$ with a velocity of $$10 m/s,$$ is exposed to an ambient of $${30^ \circ }C$$. Neglect the effect of duct construction material. For air in the range of $${20-30^ \circ }C,$$ data are as follows: thermal conductivity $$=0.025 W/m.K;$$ viscosity $$ = 18\mu Pa.s;$$ Prandtl number $$=0.73;$$ density $$= 1.2$$ $$kg/{m^3}.$$. The laminar flow Nusselt number is $$3.4$$ for constant wall temperature conditions and, for turbulent flow, $$Nu = 0.023\,\,R{e^{0.8}}\,{\Pr ^{0.33}}.$$

The Reynolds number for the flow is

An un-insulated air conditioning duct of rectangular cross section $$1\,\,m \times 0.5\,m,$$ carrying air at $${20^ \circ }C$$ with a velocity of $$10 m/s,$$ is exposed to an ambient of $${30^ \circ }C$$. Neglect the effect of duct construction material. For air in the range of $${20-30^ \circ }C,$$ data are as follows: thermal conductivity $$=0.025 W/m.K;$$ viscosity $$ = 18\mu Pa.s;$$ Prandtl number $$=0.73;$$ density $$= 1.2$$ $$kg/{m^3}.$$. The laminar flow Nusselt number is $$3.4$$ for constant wall temperature conditions and, for turbulent flow, $$Nu = 0.023\,\,R{e^{0.8}}\,{\Pr ^{0.33}}.$$

The heat transfer per meter length of the duct, in watts, is