Convection Heat Transfer
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Questions and Answers

What is the formula for convection heat transfer rate?

Qconv = hAs (Ts - T)

What is the thermal resistance of the surface against heat convection?

Rconv = 1 / (hAs)

What is the formula for radiation heat transfer rate?

Qrad = hrad As (Ts^4 - Tsurr^4)

What is the thermal resistance of a surface against radiation?

<p>Rrad = 1 / (hrad As)</p> Signup and view all the answers

What is the unit of convection heat transfer coefficient?

<p>W/m^2K</p> Signup and view all the answers

What is the unit of radiation heat transfer coefficient?

<p>W/m^2K^4</p> Signup and view all the answers

What is the formula for convection heat transfer rate in terms of convection resistance?

<p>Qconv = (Ts - T) / Rconv</p> Signup and view all the answers

What is the formula for radiation heat transfer rate in terms of radiation resistance?

<p>Qrad = (Ts - Tsurr) / Rrad</p> Signup and view all the answers

What is the concept that can be used to solve heat conduction problems without involving differential equations?

<p>thermal resistance</p> Signup and view all the answers

What is the analogy between heat conduction and electrical circuit problems?

<p>thermal resistance corresponds to electrical resistance, temperature difference corresponds to voltage, and heat transfer rate corresponds to electric current</p> Signup and view all the answers

What is the geometric shape of the plane wall considered in the problem of one-dimensional steady heat conduction?

<p>a plane wall of thickness L</p> Signup and view all the answers

What is the relationship between the temperature distribution and the distance x through the wall under steady conditions?

<p>the temperature distribution in the wall is a straight line</p> Signup and view all the answers

What is the expression for the rate of conduction heat transfer Qcond,wall through the wall?

<p>Qcond,wall = kA(T1-T2)/L</p> Signup and view all the answers

What is the thermal resistance of the wall against heat conduction, Rwall?

<p>Rwall = L/(kA)</p> Signup and view all the answers

What is the expression for the heat conduction through a plane wall in terms of thermal resistance?

<p>Qcond,wall = (T1-T2)/Rwall</p> Signup and view all the answers

What are the typical units of thermal resistance, Rwall?

<p>C/W</p> Signup and view all the answers

What are the two ways heat can be transferred in a solid?

<p>conduction and radiation</p> Signup and view all the answers

What happens to heat transfer when a fluid is still (no bulk fluid motion)?

<p>It is by conduction and possibly by radiation</p> Signup and view all the answers

What is the role of gases in radiation?

<p>They are practically transparent to radiation, except at certain wavelengths</p> Signup and view all the answers

What is the primary mode of heat transfer in a vacuum?

<p>radiation</p> Signup and view all the answers

What is the condition for approximating heat conduction as one-dimensional in a large plane wall?

<p>Heat conduction is dominant in one direction and negligible in other directions</p> Signup and view all the answers

What is the relationship between convection and conduction in a fluid?

<p>Convection is a combination of conduction and fluid motion, and conduction is a special case of convection in the absence of fluid motion</p> Signup and view all the answers

What is the primary mode of heat transfer in a liquid?

<p>conduction and absorption of radiation</p> Signup and view all the answers

Can a gas between two solid surfaces interfere with radiation?

<p>No, it acts effectively as a vacuum</p> Signup and view all the answers

What is the main assumption in the heat transfer problem of a single-pane window, considering the temperature variation?

<p>One-dimensional heat transfer, with no variation in the axial direction.</p> Signup and view all the answers

What is the thermal conductivity of the glass window in the example problem?

<p>0.78 W/m°C</p> Signup and view all the answers

What is the purpose of the heat transfer coefficients h1 and h2 in the single-pane window problem?

<p>To include the effects of radiation in the heat transfer process.</p> Signup and view all the answers

What is the temperature of the surroundings in the steam pipe problem?

<p>5°C</p> Signup and view all the answers

What is the assumption made for the thermal conductivity of the wall in the problem of steady one-dimensional heat conduction?

<p>Constant thermal conductivity</p> Signup and view all the answers

What is the thickness of the glass wool insulation in the steam pipe problem?

<p>3 cm</p> Signup and view all the answers

What is the rate of heat loss from the steam per unit length of the pipe?

<p>93.9 W</p> Signup and view all the answers

Why does adding insulation to a wall always decrease heat transfer?

<p>Because the heat transfer area A is constant, and adding insulation increases the thermal resistance of the wall without increasing the convection resistance</p> Signup and view all the answers

What happens to the convection resistance of a cylindrical pipe or a spherical shell when insulation is added?

<p>It decreases</p> Signup and view all the answers

What is the purpose of finned surfaces in heat transfer?

<p>To increase the heat transfer rate from the surface.</p> Signup and view all the answers

What is the total thermal resistance in the steam pipe problem?

<p>3.355 C/W</p> Signup and view all the answers

What is the condition required to determine the value of r2 at which Q reaches a maximum?

<p>dQ/dr2 = 0</p> Signup and view all the answers

What is the temperature of the right surface of the wall at x = L?

<p>T = -4°C</p> Signup and view all the answers

What is the effect of adding insulation to a cylindrical pipe on the conduction resistance?

<p>It increases the conduction resistance of the insulation layer</p> Signup and view all the answers

Why does the heat transfer from a pipe may increase or decrease when insulation is added?

<p>Because the effect of increasing conduction resistance dominates or is dominated by the effect of decreasing convection resistance</p> Signup and view all the answers

What is the expression for the rate of heat transfer from the insulated pipe to the surrounding air?

<p>The equation given in the problem statement</p> Signup and view all the answers

Study Notes

Convection Heat Transfer

  • The convection heat transfer rate (Qconv) can be expressed as: Qconv = hAs (Ts - T) where h is the convection heat transfer coefficient, As is the surface area, Ts is the surface temperature, and T is the fluid temperature.
  • The convection resistance (Rconv) of a surface can be expressed as: Rconv = 1 / hAs

Radiation Heat Transfer

  • The radiation heat transfer rate (Qrad) can be expressed as: Qrad = hrad As (Ts^4 - Tsurr^4) where hrad is the radiation heat transfer coefficient, As is the surface area, Ts is the surface temperature, and Tsurr is the surrounding temperature.
  • The radiation resistance (Rrad) of a surface can be expressed as: Rrad = 1 / hrad As

Heat Transfer Modes

  • Conduction, convection, and radiation are the three modes of heat transfer.
  • Conduction occurs through a stationary medium, convection occurs through a moving fluid, and radiation occurs through electromagnetic waves.
  • The mode of heat transfer depends on the presence or absence of fluid motion and the temperature differences between surfaces.

One-Dimensional Heat Conduction

  • Heat conduction through a large plane wall, cylinder, or sphere can be approximated as one-dimensional.
  • The thermal resistance of a plane wall can be expressed as: Rwall = L / kA, where L is the thickness of the wall, k is the thermal conductivity, and A is the cross-sectional area.
  • The rate of heat conduction (Qcond) can be expressed as: Qcond = kA (T1 - T2) / L, where T1 and T2 are the temperatures on either side of the wall.

Thermal Resistance Concept

  • Thermal resistance is analogous to electrical resistance, with temperature difference as the driving force and heat transfer rate as the resulting flow.
  • Thermal resistance can be used to analyze heat conduction problems in various geometries, including plane walls, cylinders, and spheres.

Critical Radius of Insulation

  • Adding insulation to a wall always decreases heat transfer, but adding insulation to a cylindrical pipe or spherical shell can increase or decrease heat transfer, depending on the dominance of conduction and convection resistances.
  • The critical radius of insulation is the radius at which the heat transfer rate reaches a maximum or minimum.

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Quiz about convection heat transfer, including the equation for heat flow and convection heat transfer coefficient.

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