HW# 2 /Tutorial # 2WRF Chapter 16; WWWR Chapter 17

ID Chapter 3

• Tutorial #2• WRF#16.2;WWWR#17.13,

WRF#16.1; WRF#16.12; WRF#17.39; WRF#16.22.

• To be discussed during the week 25 Jan. – 29 Jan., 2016.

• By either volunteer or class list.

• Homework # 2 (Self practice)

• WWWR #17.15, 17.26

• ID # 3.57

HW# 2 /Tutorial # 2Hints / Corrections

• Tutorial #2• WWWR• #17.39: Line 2: The fins are made of

aluminum, they are 0.3cm thick each.

• Homework # 2• WWWR • #17.15: You may model the whole heat

transfer process as the series/parallel connection of 5 resistors: R1 internal convective transfer + R2: Plaster + R3 (Pine Studs) and R4 (Fiberglass) in parallel + R5: External convective transfer. The problem does not specify which is the “inside” and which is the “outside”. Please fix the side next to the plaster to be the inside (R1: hence a temperature of 25oC is maintained here) and the opposite side to be outside (R5: a temperature of -10 degree C is specified here).

• # 17.26: Generate heat uniformly at a rate of 5170 kJ/sm3. the fuel is placed in an environment having a temperature of 370K.

Steady-State Conduction

One-Dimensional Conduction

02 T

Steady-state conduction, no internal generation of energy

0id dTx

dx dx

For one-dimensional, steady-state transfer by conductioni = 0 rectangular coordinatesi = 1 cylindrical coordinatesi = 2 spherical coordinates

1/Rc=1/Ra+1/Rb

, (m)cr cylinder

kr

h

Adapted from Heat and Mass Transfer – A Practical Approach, Y.A. Cengel, Third Edition, McGraw Hill 2007.

• Thus, insulating the pipe • may actually increase the • rate of heat transfer instead • of decreasing it.

For steady-state conduction in the x direction without internal generation of energy, the equation which applies is

Where k may be a function of T.

In many cases the thermal conductivity may be a linear function temperature over a considerable range. The equation of such a straight-line function may be expressed by

k = ko(1 + ßT)

Where koand ß are constants for a particular material

One-Dimensional Conduction With Internal Generation of Energy

Plane Wall with Variable Energy Generation

q = qL [ 1 + ß (T - T L)]. .

The symmetry of the temperature distribution requires a zero temperature gradient at x = 0.The case of steady-state conduction in the x direction in a stationary solid with constant thermal conductivity becomes

Detailed derivation for the transformation= C + s

Detailed Derivation for Equations 17-25

Courtesy by all CN5 Grace Mok, 2003-2004

Detailed Derivation for Equations 17-25

Courtesy by all CN5 Grace Mok, 2003-2004

Heat Transfer from Finned Surfaces

• Temperature gradient dT/dx,• Surface temperature, T,• Are expressed such that T is a function of x only.• Newton’s law of cooling

• Two ways to increase the rate of heat transfer:– increasing the heat transfer coefficient,– increase the surface area fins

• Fins are the topic of this section.

Adapted from Heat and Mass Transfer – A Practical Approach, Y.A. Cengel, Third Edition, McGraw Hill 2007.

conv s sQ hA T T

Heat transfer from extended surfaces

For constant cross section and constant thermal conductivity

Where

• Equation (A) is a linear, homogeneous, second-order differential equation with constant coefficients.

• The general solution of Eq. (A) is

• C1 and C2 are constants whose values are to be determined from the boundary conditions at the base and at the tip of the fin.

22

20

dm

dx

(A)

2 ; ; cc

hpT T m A A

kA

1 2( ) mx mxx C e C e (B)

Boundary Conditions

Several boundary conditions are typically employed:• At the fin base

– Specified temperature boundary condition, expressed as: (0)= b= Tb-T∞

• At the fin tip1. Specified temperature

2. Infinitely Long Fin

3. Adiabatic tip

4. Convection (and

combined convection).

Adapted from Heat and Mass Transfer – A Practical Approach, Y.A. Cengel, Third Edition, McGraw Hill 2007.

How to derive the functional dependence of for a straight fin with variable cross section area Ac = A = A(x)?

General Solution for Straight Fin with Three Different Boundary Conditions

In set(a)Known temperature at x = L

In set(b)Temperature gradient is zero at x = L

In set(c)Heat flow to the end of an extended surface by conduction be equal to that leaving this position by convection.

Detailed Derivation for Equations 17-36 (Case a).Courtesy by CN3 Yeong Sai Hooi 2002-2003

Detailed Derivation for Equations 17-38 (Case b for extended surface heat transfer). Courtesy by CN3 Yeong Sai Hooi, 2002-2003

Detailed Derivation for Equations 17-40 (Case c for extended surface heat transfer). Courtesy by all CN4 students, presented by Loo Huiyun, 2002-2003

Detailed Derivation for Equations 17-46 (Case c for extended surface heat transfer). Courtesy by all CN4 students, presented by Loo Huiyun, 2002-2003

Infinitely Long Fin (Tfin tip=T) Adapted from Heat and Mass Transfer – A Practical Approach, Y.A. Cengel, Third Edition, McGraw Hill 2007.

• For a sufficiently long fin the temperature at the fin tip approaches the ambient temperature

Boundary condition: (L→∞)=T(L)-T∞=0

• When x→∞ so does emx→∞

C1=0

• @ x=0: emx=1 C2= b

• The temperature distribution:

• heat transfer from the entire fin

/( )cx hp kAmx

b

T x Te e

T T

0

c c bx

dTq kA hpkA T T

dx

Fin Efficiency Adapted from Heat and Mass Transfer –

A Practical Approach, Y.A. Cengel, Third Edition, McGraw Hill 2007.

• To maximize the heat transfer from a fin the temperature of the fin should be uniform (maximized) at the base value of Tb

• In reality, the temperature drops along the fin, and thus the heat transfer from the fin is less

• To account for the effect we define

a fin efficiency

or

,max

finfin

fin

q

q Actual heat transfer rate from the fin

Ideal heat transfer rate from the finif the entire fin were at base temperature

,max ( )fin fin fin fin fin bq q hA T T

Fin Efficiency Adapted from Heat and Mass Transfer –

A Practical Approach, Y.A. Cengel, Third Edition, McGraw Hill 2007.

• For constant cross section of very long fins:

• For constant cross section with adiabatic tip:

,

,max

1 1fin c b clong fin

fin fin b

q hpkA T T kA

q hA T T L hp mL

,

,max

tanh

tanh

fin c badiabatic fin

fin fin b

q hpkA T T mL

q hA T T

mL

mL

Afin = P*L

q

,max

fin

fin

q

q

Fin Effectiveness Adapted from Heat and Mass Transfer –

A Practical Approach, Y.A. Cengel, Third Edition, McGraw Hill 2007.

• The performance of the fins is judged on the basis of the enhancement in heat transfer relative to the no-fin case.

• The performance of fins is expressed

in terms of the fin effectiveness fin

defined as

fin finfin

no fin b b

q q

q hA T T

Heat transfer rate

from the surface of area Ab

Heat transfer rate from the fin of base

area Ab

finq

no finq

finfin

no fin

q

q

Governing Differential Equation for Circular Fin:Temperature variation in the R (radial) direction only!

T = T(r)

(RL-Ro)

Problem: Water and air are separated by a mild-steel plane wall. I is proposed to increase the heat-transfer rate between these fluids by adding Straight rectangular fins of 1.27mm thickness, and 2.5-cm length, spaced 1.27 cm apart.

Two and Three - Dimensional Systems

Analytical SolutionAnalytical solution to any transfer problem must satisfy the differential equation •describing the process•Prescribed boundary conditions

The steady-state temperature distribution in the plate of constant thermal conductivity must satisfy the differential equation

We obtain an expression in which the variables are separated

(17-57)

Detailed Derivation for Equations 17-57

Courtesy by all CN6 Leow Sheue Ling, 2003-2004