Heat and Mass Transfer Chapter 2 Extended Surfaces

What is the purpose of using fins in a particular heat transfer system?

• To decrease rate of heat transfer

• To increase rate of heat transfer

• To maintain rate of heat transfer at a constant rate

• cannot say

Temperature at the end tip of the fin having uniform cross-sectional area is

• maximum

• minimum

• similar to the heat generation temperature

• unpredictable

Consider that heat transfer is taking place through a fin having circular cross-sectional area, one dimensionally as shown in figure.The rate of heat transfer by conduction into a section at x is equal to

• sum of rate of heat transfer by convection out of the element (x+dx) and heat transfer by convection from the surface between x to (x+dx)

• sum of rate of heat transfer by conduction out of the element (x+dx) and heat transfer by conduction from the surface between x to (x+dx)

• sum of rate of heat transfer by conduction out of the element (x+dx) and heat transfer by convection from the surface between x to (x+dx)

• none of the above

In the process of heat transfer through extended surfaces or fins, the entire surface area is at

• the same constant temperature

• different temperatures

• maximum base temperature

• minimum temperature

Aluminum is used as a fin material because

• it has higher convection heat transfer coefficient

• it has higher thermal conductivity

• it has lower convection heat transfer coefficient

• it has lower thermal conductivity

Which medium of surrounding is better for fin effectiveness?

• gas medium

• liquid medium

• fins have same effectiveness in both the gas and liquid mediums

• none of the above

What is efficiency of fin?

• the ratio of actual heat transferred from fin area to the heat which would be transferred if entire fin area was at base temperature

• the heat which would be transferred if entire fin area was at base temperature to the ratio of actual heat transferred from fin area

• the heat which would be transferred if entire fin area was at minimum temperature to the ratio of actual heat transferred from fin area

• . the ratio of actual heat transferred from fin area to the heat which would be transferred if entire fin area was at minimum temperature

What is the effect of thermal conductivity k on fin effectiveness?

• fin is effective for smaller value of thermal conductivity k

• fin is effective for larger value of thermal conductivity k

• thermal conductivity k does not affect the fin effectiveness

• cannot say

What is the effect of convective heat transfer coefficient h on fin effectiveness?

• fin is effective if the value of convective heat transfer coefficient h is small

• fin is effective if the value of convective heat transfer coefficient h is large

• fin effectiveness does not affected by the value of convective heat transfer coefficient h

• none of the above

A fin of uniform cross-section A and perimeter P has a base temperature T_s and it is exposed to fluid having temperature T_∞ How is the temperature distribution in the fin given?

Where,

m = √(hP/kA)

h = convective heat transfer coefficient

k = thermal conductivity of fin

T = Temperature of the fin at the distance x from the base of fin and the constants C₁ and C₂ depend on the boundary conditions

• T – T_∞ = C₁e^mx – C₂e^{– mx}

• T – T_∞ = C₁e^mx + C₂e^{– mx}

• T + T_∞ = C₁e^mx – C₂e^{– mx}

• T + T_∞ = C₁e^mx + C₂e^{– mx}