TRANSIENT HEAT TRANSIENT HEAT CONDITIONCONDITION

1

Nazaruddin SinagaNazaruddin Sinaga

Laboratorium Efisiensi dan Konservasi EnergiLaboratorium Efisiensi dan Konservasi EnergiUniversitas DiponegoroUniversitas Diponegoro

CONTENTSCONTENTS

1. Lumped Systems Analysis

2. Transient Heat Conduction in Large Plane Walls, Long Cylinders, and Spheres with Spatial Effects

3. Transient Heat Conduction in Semi-Infinite Solids

4. Transient Heat Conduction in Multidimensional Systems

Transient Conduction

Transient Conduction• A heat transfer process for which the temperature varies with time, as well as location within a solid.

• It is initiated whenever a system experiences a change in operating conditions and proceeds until a new steady state (thermal equilibrium) is achieved.

• It can be induced by changes in:– surface convection conditions ( ), h,T

• Solution Techniques

– The Lumped Capacitance Method– Exact Solutions– The Finite-Difference Method

– surface radiation conditions ( ),r surh ,T

– a surface temperature or heat flux, and/or

– internal energy generation.

Lumped Capacitance Method

The Lumped Capacitance Method

• Based on the assumption of a spatially uniform temperature distribution throughout the transient process.

• Why is the assumption never fully realized in practice?

• General Lumped Capacitance Analysis:

Consider a general case, which includes convection, radiation and/or an applied heat flux at specified surfaces as well as internal energy generation

s,c s,r s,hA ,A ,A ,

Hence . ,T r t T t

Lumped Capacitance Method (cont.)

First Law:

stin out g

dE dTc E E E

dt dt

• Assuming energy outflow due to convection and radiation and with inflow due to an applied heat flux ,sq

, , , gs s h s c r s r sur

dTc q A hA T T h A T T E

dt

• Is this expression applicable in situations for which convection and/or radiation provide for energy inflow?

• May h and hr be assumed to be constant throughout the transient process?

• How must such an equation be solved?

Special Case (Negligible Radiation

• Special Cases (Exact Solutions, ) 0 iT T

Negligible Radiation , / :T T b a

, ,/ /gs c s s ha hA c b q A E c

The non-homogeneous differential equation is transformed into a homogeneous equation of the form:

da

dt

Integrating from t=0 to any t and rearranging,

/exp 1 exp

i i

T T b aat at

T T T T

(5.25)

To what does the foregoing equation reduce as steady state is approached?

How else may the steady-state solution be obtained?

Special Case (Convection)

Negligible Radiation and Source Terms , 0, 0 :gr sh h E q

,s c

dTc hA T T

dt (5.2)

, is c

t

o

c d

hAdt

,s c

i i

hAT Texp t

T T c

expt

t

The thermal time constant is defined as

,

1t

s c

chA

(5.7)

ThermalResistance, Rt

Lumped ThermalCapacitance, Ct

The change in thermal energy storage due to the transient process ist

outsto

E Q E dt

,

t

s co

hA dt 1 expit

tc

(5.8)

Special Case (Radiation)

Negligible Convection and Source Terms , 0, 0 :gr sh h E q

Assuming radiation exchange with large surroundings,

4 4,s r sur

dTc A T T

dt

,

4 4i

s r T

surTo

tA

c

dTT T

dt

3,

1 1n4

sur sur i

s r sur sur sur i

T T T Tct n

A T T T T T

(5.18)

Result necessitates implicit evaluation of T(t).

1 12 tan tan i

sur sur

TT

T T

Biot Number The Biot Number and Validity ofThe Lumped Capacitance Method

• The Biot Number: The first of many dimensionless parameters to be considered.

Definition:chL

Bik

thermal conductivity of t soe dh lik

of the solid ( / or coordinate

associated with maximum spati

characteri

al temp

stic

erature differenc

n

e)

le gthc sL A

Physical Interpretation:

/

/

1/ hc s cond solid

s conv solid fluid

L kA R TBi

A R T

Criterion for Applicability of Lumped Capacitance Method:

1Bi

h convection or radiation coefficient

Problem: Thermal Energy Storage

Problem 5.11: Charging a thermal energy storage system consistingof a packed bed of aluminum spheres.

KNOWN: Diameter, density, specific heat and thermal conductivity of aluminum spheres used in packed bed thermal energy storage system. Convection coefficient and inlet gas temperature.

FIND: Time required for sphere at inlet to acquire 90% of maximum possible thermal energy and the corresponding center temperature.

Alum inum sphere D = 75 m m ,

T = 25 Ci oGas

T Cg,i o= 300

h = 75 W /m -K2

= 2700 kg/m 3

k = 240 W /m -Kc = 950 J/kg-K

Schematic:

Problem: Thermal Energy Storage (cont.)

ASSUMPTIONS: (1) Negligible heat transfer to or from a sphere by radiation or conduction due to contact with other spheres, (2) Constant properties.

ANALYSIS: To determine whether a lumped capacitance analysis can be used, first compute Bi = h(ro/3)/k = 75 W/m2K (0.025m)/150 W/mK = 0.013 <<1.

Hence, the lumped capacitance approximation may be made, and a uniform temperature may be assumed to exist in the sphere at any time.

From Eq. 5.8a, achievement of 90% of the maximum possible thermal energy storage corresponds to

stt

i

E0.90 1 exp t /

cV

tt ln 0.1 427s 2.30 984s

From Eq. (5.6), the corresponding temperature at any location in the sphere is g,i i g,iT 984s T T T exp 6ht / Dc

2 3T 984s 300 C 275 C exp 6 75 W / m K 984s / 2700 kg / m 0.075m 950 J / kg K

If the product of the density and specific heat of copper is (c)Cu 8900 kg/m3 400 J/kgK = 3.56 106 J/m3K, is there any advantage to using copper spheres of equivalent diameter in lieu of aluminum spheres?

Does the time required for a sphere to reach a prescribed state of thermal energy storage change with increasing distance from the bed inlet? If so, how and why?

T 984s 272.5 C

3

t s 2

2700 kg / m 0.075m 950 J / kg KVc / hA Dc / 6h 427s.

6 75 W / m K

Problem: Furnace Start-up

Problem 5.15: Heating of coated furnace wall during start-up.

KNOWN: Thickness and properties of furnace wall. Thermal resistance of ceramic coating on surface of wall exposed to furnace gases. Initial wall temperature.

FIND: (a) Time required for surface of wall to reach a prescribed temperature, (b) Corresponding value of coating surface temperature.

Schematic:

Problem: Furnace Start-up

ASSUMPTIONS: (1) Constant properties, (2) Negligible coating thermal capacitance, (3) Negligible radiation.

PROPERTIES: Carbon steel: = 7850 kg/m3, c = 430 J/kgK, k = 60 W/mK.

ANALYSIS: Heat transfer to the wall is determined by the total resistance to heat transfer from the gas to the surface of the steel, and not simply by the convection resistance.

11

1 2 2 2tot f 2

1 1U R R 10 m K/W 20 W/m K.

h 25 W/m K

2UL 20 W/m K 0.01 mBi 0.0033 1

k 60 W/m K

and the lumped capacitance method can be used.

(a) From Eqs. (5.6) and (5.7),

t t ti

T Texp t/ exp t/R C exp Ut/ Lc

T T

3

2i

7850 kg/m 0.01 m 430 J/kg KT TLc 1200 1300t ln ln

U T T 300 130020 W/m K

t 3886s 1.08h.

Hence, with

Problem: Furnace Start-up (cont.)

(b) Performing an energy balance at the outer surface (s,o),

s,o s,o s,i fh T T T T / R

2 -2 2s,i f

s,o 2f

hT T / R 25 W/m K 1300 K 1200 K/10 m K/WT

h 1/ R 25 100 W/m K

s,oT 1220 K.

How does the coating affect the thermal time constant?

Transient Heat Conduction in Large Transient Heat Conduction in Large Plane Walls, Long Cylinders, and Plane Walls, Long Cylinders, and

Spheres with Spatial EffectsSpheres with Spatial Effects

In this section, we consider the variation of temperature with time and position in one-dimensional problems such as those associated with a large plane wall, a long cylinder, and a sphere.

Consider a plane wall of thickness 2L, a long cylinder of radius ro, and a sphere of radius ro initially at a uniform temperature Ti, as shown in figure.

At time t = 0, each geometry is placed in a large medium that is at a constant temperature T and kept in that medium for t = 0.

Heat transfer takes place between these bodies and their environments by convection with a uniform and constant heat transfer coefficient h. Note that all three cases possess geometric and thermal symmetry: the plane wall is symmetric about its center plane (x = 0), the cylinder is symmetric about its centerline (r = 0), and the sphere is symmetric about its center point (r = 0).

We neglect radiation heat transfer between these bodies and their surrounding surfaces, or incorporate the radiation effect into the convection heat transfer coefficient h.

The variation of the temperature profile with time in the plane wall is illustrated in figure.

When the wall is first exposed to the surrounding medium at T∞ < Ti at t = 0, the entire wall is at its initial temperature Ti. But the wall temperature at and near the surfaces starts to drop as a result of heat transfer from the wall to the surrounding medium.

Transient temperature profiles in a plane wall exposed to convection from its surfaces.

This creates a temperature gradient in the wall and initiates heat conduction from the inner parts of the wall toward its outer surfaces.

Note that the temperature at the center of the wall remains at Ti until t = t2, and that the temperature profile within the wall remains symmetric at all times about the center plane.

The temperature profile gets flatter and flatter as time passes as a result of heat transfer, and eventually becomes uniform at T = T∞. that is, the wall reaches thermal equilibrium with its surroundings.

At that point, the heat transfer stops since there is no longer a temperature difference.

The formulation of the problems for the determination of the one dimensional transient temperature distribution T(x, t) in a wall results in a partial differential equation, which can be solved using advanced mathematical techniques.

The solution, however, normally involves infinite series, which are inconvenient and time-consuming to evaluate. Therefore, there is clear motivation to present the solution in tabular or graphical form.

The solution involves the parameters x, L, t, k, , h, Ti, and T∞, which are too many to make any graphical presentation of the results practical.

In order to reduce the number of parameters, we nondimensionalize the problem by defining the following dimensionless quantities:

Dimensionless temperature:

Dimensionless distance from the center:

Dimensionless heat transfer coefficient :

Dimensionless time:

The nondimensionalization enables us to present the temperature in terms of three parameters only: X, Bi, and .

This makes it practical to present the solution in graphical form.

The dimensionless quantities defined above for a plane wall can also be used for a cylinder or sphere by replacing the space variable x by r and the half-thickness L by the outer radius ro.

Note that the characteristic length in the definition of the Biot number is taken to be the half-thickness L for the plane wall, and the radius ro for the long cylinder and sphere instead of V/A used in lumped system analysis.

The one-dimensional transient heat conduction problem just described can be solved exactly for any of the three geometries, but the solution involves infinite series, which are difficult to deal with.

However, the terms in the solutions converge rapidly with increasing time, and for > 0.2, keeping the first term and neglecting all the remaining terms in the series results in an error under 2 percent.

We are usually interested in the solution for times with > 0.2, and thus it is very convenient to express the solution using this one- term approximation, given as

Where the constants A1 and 1 are functions of the Bi number only, and their values are listed in Table 4–1 against the Bi number for all three geometries.

The function J0 is the zeroth-order Bessel function of the first kind, whose value can be determined from Table 4–2. Noting that cos (0) = J0 (0) = 1 and the limit of (sin x)/x is also 1, these relations simplify to the next ones at the center of a plane wall, cylinder, or sphere:

Once the Bi number is known, the above relations can be used to determine the temperature anywhere in the medium.

The determination of the constants A1 and 1 usually requires interpolation.

For those who prefer reading charts to interpolating, the relations above are plotted and the one-term approximation solutions are presented in graphical form, known as the transient temperature charts.

Note that the charts are sometimes difficult to read, and they are subject to reading errors. Therefore, the relations above should be preferred to the charts.

The transient temperature charts in Figs. 4–13, 4–14, and 4–15 for a large plane wall, long cylinder, and sphere were presented by M. P. Heisler in 1947 and are called Heisler charts.

They were supplemented in 1961 with transient heat transfer charts by H. Gröber. There are three charts associated with each geometry:

The first chart is to determine the temperature To at the center of the geometry at a given time t.

The second chart is to determine the temperature at other locations at the same time in terms of To.

The third chart is to determine the total amount of heat transfer up to the time t.

These plots are valid for > 0.2.

FIG. 4–13 Transient temperature and heat transfer charts for a plane wall of thickness 2L initially at a uniform temperature Ti subjected to convection from both sides to an environment at temperature T with a convection coefficient of h.

(a) Midplane temperature (from M. P. Heisler)

(b) Temperature distribution (from M. P. Heisler)

(c) Heat transfer (from H. Gröber et al.)

The temperature of the body changes from the initial temperature Ti to the temperature of the surroundings T∞ at the end of the transient heat conduction process.

Thus, the maximum amount of heat that a body can gain (or lose if Ti > T∞) is simply the change in the energy content of the body. That is,

Thus, Qmax represents the amount of heat transfer for t → ∞ .The amount of heat transfer Q at a finite time t will obviously be less than this maximum.

The fraction of heat transfer can also be determined from these relations, which are based on the one-term approximations already discussed:

The use of the Heisler/Gröber charts and the one-term solutions already discussed is limited to the conditions specified at the beginning of this section:

othe body is initially at a uniform temperature;othe temperature of the medium surrounding the body and the convection heat transfer coefficient are constant and uniform;othere is no energy generation in the body.

FIGURE 4–17The fraction of total heat transfer Q/Qmax up to a specified time t isdetermined using the Gröber charts.

To understand the physical significance of the Fourier number , we express it as (Fig. 4–18)

FIGURE 4–18Fourier number at time t can be viewed as the ratio of the rate of heatconducted to the rate of heat stored at that time.

TRANSIENT HEAT CONDUCTIONTRANSIENT HEAT CONDUCTIONIN SEMI-INFINITE SOLIDSIN SEMI-INFINITE SOLIDS

A semi-infinite solid is an idealized body that has a single plane surface and extends to infinity in all directions, as shown in Fig. 4–22.

This idealized body is used to indicate that the temperature change in the part of the body in which we are interested (the region close to the surface) is due to the thermal conditions on a single surface.

Consider a semi-infinite solid that is at a uniform temperature Ti. At time t = 0, the surface of the solid at x = 0 is exposed to convection by a fluid at a constant temperature T, with a heat transfer coefficient h.

This problem can be formulated as a partial differential equation, which can be solved analytically for the transient temperature distribution T(x, t).

The solution obtained is presented in Fig. 4–23 graphically for the nondimensionalized temperature defined as

FIGURE 4–23 Variation of temperature with position and time in a semi-infinite solid initially at Ti subjected to convection to an environment at T with a convection heat transfer coefficient of h (from P. J. Schneider, Ref. 10).

The exact solution of the transient one-dimensional heat conduction problem in a semi-infinite medium that is initially at a uniform temperature of Ti and is suddenly subjected to convection at time t = 0 has been obtained, and is expressed as

where the quantity erfc ( ) is the complementary error function, defined as

Despite its simple appearance, the integral that appears in the above relation cannot be performed analytically.

Therefore, it is evaluated numerically for different values of , and the results are listed in Table 4–3.