MENG 335: HEAT TRANSFER Quiz#1 A 29 February 2016 Student name: Student number:

ID: 1718701 • Letter: M

Question

MENG 335: HEAT TRANSFER Quiz#1 A 29 February 2016 Student name: Student number: 1. (8pts) TRUE/FALSE Heat flux. q" is the heat transfer rate .Q, times the area A. The thermal conductivity of a metal is constant and not a function oftemperature. At steady state, the temperature of material, Try , is constant. Heat transfer from a cylindrical pipe to the surroundings involves the surface arca - of the pipe ( r'), where ris he radius of the pipe. 2. (10 pts) I n the diagram below for a material with thermal conductivity k, what is the heat transfer rate Q by conduction from node I to node 2? Leave your answer in terms of. T1-T2. Ax, y. Az and k. 11.1, 3. (4pts) Mark an (X) by the following statements that will increase the heat transfer rate. _ placing a layer of air between two glass for a double-paned window (vs. a single- - paned window) increasing the temperature difference between two points. 4. (3 pts) For 1-dimensional, unsteady state heat transfer with no heat generated in the walls, and constant thermal conductivity what term(s) of the energy balance equation can be ignored? Write the final form of this equation. OT (WHEN COMPLETED, TURN IN TO RECEIVE OPEN B0OK PORTION)

Explanation / Answer

1.a. FALSE

Explanation:

According to Fouries law rate of Heat conduction in x- direction is proportional to area measured normal to the direction of heat flow and to the temperature gradient in that x-direction

= -kA

The constant of proportionality is called the coefficient of thermal conductivity k

Heat flux: The quantity of heat transfer per unit time per unit area of isothermal surface

= -k

b. False

Explanation:

Thermal conductivity (k) is a physical property of a substance and is defined as an ability of a substance to conduct heat. k is constant for metal substances

Examples:

Substance

Silver

Copper

Aluminum

Mild steel

Water

Air

Wood

Thermal conductivity(W/mK)

407

386

175

37.2

0.51

0.022

0.15

For many engineering problems metals are often consider to possess a constant thermal conductivity (isotropic).

But it is primarily function of temperature and/or position and nature of substance.

So, k is function of temperature.

c .True

In steady state condition any property does not change with time.

So in steady state temperature is independent of time.

d. False

Because Heat transfer for cylindrical pipe to surroundings involves surface area of pipe i.e Q = - k

Where = surface area of pipe = 2rL

L = length of pipe

3. Explanation

1.By placing air between two glasses resists the heat transfer rate so it causes the decrease in heat transfer rate by compared with single glass heat transfer rate(X)

2. Because the heat transfer rate is proportional to the temperature gradient the heat transfer rate increases with temperature difference

4. Explanation:

The energy balance for little element is obtained from the first law of thermodynamics:

Net heat conduct into element dx dy dz per unit time + Internal heat generated per unit time= increase in internal energy per unit time + work done by element per unit time

From the given equation some terms are cancelled out why because,

so finally the equation becomes

6. Explanation:

The heat transfer process

3.b/w plastic flask to cover wall is convection

4.b/w cover walls are conduction

5. cover wall to surroundings is convection

To cool the hot coffee quickly we have to select the material which have low resistance for heat transfer