39 real Heat Transfer questions from the Mechanical Core bank, as asked in Indian campus drives and tech interviews. Every question has a verified answer and an AI-tutor explanation on placd — free to start.
1. What is Fourier's law?
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A.outer radius k/h for a cylinder below which adding insulation increases heat loss instead of reducing it
B.ratio of internal conduction resistance to surface convection resistance hL/k, with lumped-capacitance analysis valid below 0.1
C.conductive heat flux is proportional to the negative temperature gradient, with thermal conductivity as the constant
D.the ratio of thermal conductivity to the product of density and specific heat, governing how fast a temperature disturbance propagates
A.Fourier's law — the ratio of momentum diffusivity to thermal diffusivity, which fixes the relative thickness of the velocity and thermal boundary layers
B.Fourier's law — ratio of internal conduction resistance to surface convection resistance hL/k, with lumped-capacitance analysis valid below 0.1
C.Fourier's law — the ratio of thermal conductivity to the product of density and specific heat, governing how fast a temperature disturbance propagates
D.Fourier's law — conductive heat flux is proportional to the negative temperature gradient, with thermal conductivity as the constant
A.Newton's law of cooling — heat exchanger rating using UA times the log-mean temperature difference, with counter-flow giving a higher value than parallel flow for the same terminal temperatures
B.Newton's law of cooling — convective heat transfer rate equals hA times the temperature difference between the surface and the bulk fluid
C.Newton's law of cooling — the ratio of convective to conductive heat transfer across a boundary, so a value of one means pure conduction
D.Newton's law of cooling — conductive heat flux is proportional to the negative temperature gradient, with thermal conductivity as the constant
A.Stefan-Boltzmann law — a black body emits radiation at σT⁴ per unit area, with σ = 5.67 × 10⁻⁸ W/m²K⁴
B.Stefan-Boltzmann law — ratio of internal conduction resistance to surface convection resistance hL/k, with lumped-capacitance analysis valid below 0.1
C.Stefan-Boltzmann law — peak of the boiling curve beyond which a vapour film blankets the surface and wall temperature jumps, the burnout limit for boiler tubes
D.Stefan-Boltzmann law — outer radius k/h for a cylinder below which adding insulation increases heat loss instead of reducing it
11. Which term means: "ratio of internal conduction resistance to surface convection resistance hL/k, with lumped-capacitance analysis valid below 0.1"?
A.Biot number — peak of the boiling curve beyond which a vapour film blankets the surface and wall temperature jumps, the burnout limit for boiler tubes
B.Biot number — ratio of heat transfer with a fin to that from the bare base area, with fins justified only when it exceeds about 2
C.Biot number — the ratio of convective to conductive heat transfer across a boundary, so a value of one means pure conduction
D.Biot number — ratio of internal conduction resistance to surface convection resistance hL/k, with lumped-capacitance analysis valid below 0.1
A.Critical radius of insulation — heat exchanger rating using UA times the log-mean temperature difference, with counter-flow giving a higher value than parallel flow for the same terminal temperatures
B.Critical radius of insulation — outer radius k/h for a cylinder below which adding insulation increases heat loss instead of reducing it
C.Critical radius of insulation — the ratio of momentum diffusivity to thermal diffusivity, which fixes the relative thickness of the velocity and thermal boundary layers
D.Critical radius of insulation — a black body emits radiation at σT⁴ per unit area, with σ = 5.67 × 10⁻⁸ W/m²K⁴
A.a black body emits radiation at σT⁴ per unit area, with σ = 5.67 × 10⁻⁸ W/m²K⁴
B.ratio of heat transfer with a fin to that from the bare base area, with fins justified only when it exceeds about 2
C.heat exchanger rating using UA times the log-mean temperature difference, with counter-flow giving a higher value than parallel flow for the same terminal temperatures
D.conductive heat flux is proportional to the negative temperature gradient, with thermal conductivity as the constant
A.Fin effectiveness — sizing approach used when outlet temperatures are unknown, relating effectiveness to UA/Cmin and the capacity ratio
B.Fin effectiveness — convective heat transfer rate equals hA times the temperature difference between the surface and the bulk fluid
C.Fin effectiveness — peak of the boiling curve beyond which a vapour film blankets the surface and wall temperature jumps, the burnout limit for boiler tubes
D.Fin effectiveness — ratio of heat transfer with a fin to that from the bare base area, with fins justified only when it exceeds about 2
A.ratio of heat transfer with a fin to that from the bare base area, with fins justified only when it exceeds about 2
B.heat exchanger rating using UA times the log-mean temperature difference, with counter-flow giving a higher value than parallel flow for the same terminal temperatures
C.the ratio of momentum diffusivity to thermal diffusivity, which fixes the relative thickness of the velocity and thermal boundary layers
D.sizing approach used when outlet temperatures are unknown, relating effectiveness to UA/Cmin and the capacity ratio
20. Which term means: "heat exchanger rating using UA times the log-mean temperature difference, with counter-flow giving a higher value than parallel flow for the same terminal temperatures"?
A.LMTD method — heat exchanger rating using UA times the log-mean temperature difference, with counter-flow giving a higher value than parallel flow for the same terminal temperatures
B.LMTD method — ratio of heat transfer with a fin to that from the bare base area, with fins justified only when it exceeds about 2
C.LMTD method — sizing approach used when outlet temperatures are unknown, relating effectiveness to UA/Cmin and the capacity ratio
D.LMTD method — a black body emits radiation at σT⁴ per unit area, with σ = 5.67 × 10⁻⁸ W/m²K⁴
A.heat exchanger rating using UA times the log-mean temperature difference, with counter-flow giving a higher value than parallel flow for the same terminal temperatures
B.peak of the boiling curve beyond which a vapour film blankets the surface and wall temperature jumps, the burnout limit for boiler tubes
C.sizing approach used when outlet temperatures are unknown, relating effectiveness to UA/Cmin and the capacity ratio
D.the ratio of momentum diffusivity to thermal diffusivity, which fixes the relative thickness of the velocity and thermal boundary layers
A.Effectiveness-NTU method — peak of the boiling curve beyond which a vapour film blankets the surface and wall temperature jumps, the burnout limit for boiler tubes
B.Effectiveness-NTU method — ratio of heat transfer with a fin to that from the bare base area, with fins justified only when it exceeds about 2
C.Effectiveness-NTU method — the ratio of convective to conductive heat transfer across a boundary, so a value of one means pure conduction
D.Effectiveness-NTU method — sizing approach used when outlet temperatures are unknown, relating effectiveness to UA/Cmin and the capacity ratio
26. Which term means: "peak of the boiling curve beyond which a vapour film blankets the surface and wall temperature jumps, the burnout limit for boiler tubes"?
A.Critical heat flux — convective heat transfer rate equals hA times the temperature difference between the surface and the bulk fluid
B.Critical heat flux — peak of the boiling curve beyond which a vapour film blankets the surface and wall temperature jumps, the burnout limit for boiler tubes
C.Critical heat flux — ratio of internal conduction resistance to surface convection resistance hL/k, with lumped-capacitance analysis valid below 0.1
D.Critical heat flux — conductive heat flux is proportional to the negative temperature gradient, with thermal conductivity as the constant
A.heat exchanger rating using UA times the log-mean temperature difference, with counter-flow giving a higher value than parallel flow for the same terminal temperatures
B.ratio of heat transfer with a fin to that from the bare base area, with fins justified only when it exceeds about 2
C.conductive heat flux is proportional to the negative temperature gradient, with thermal conductivity as the constant
D.the ratio of convective to conductive heat transfer across a boundary, so a value of one means pure conduction
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