Heat Transfer Coefficient

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Heat transfer coefficient

From Wikipedia, the free encyclopedia

The heat transfer coefficient, in thermodynamics and in mechanical and chemical engineering, is used in calculating the heat transfer, typically by convection or phase change between a fluid and a solid:

where

q = heat flow in input or lost heat flow , J/s = W h = heat transfer coefficient, W/(m2K)

A = heat transfer surface area, m2

ΔT

= difference in temperature between the solid surface and surrounding fluid area, K

From the above equation, the heat transfer coefficient is the proportionality coefficient between the heat flux that is a heat flow per unit area, q/A, and the thermodynamic driving force for the flow of heat (i.e., the temperature difference, ΔT).

The heat transfer coefficient has SI units in watts per meter squared-kelvin: W/(m2K). Heat transfer coefficient is the inverse of thermal insulance.

There are numerous methods for calculating the heat transfer coefficient in different heat transfer modes, different fluids, flow regimes, and under different thermohydraulic conditions. Often it can be estimated by dividing the thermal conductivity of the convection fluid by a length scale. The heat transfer coefficient is often calculated from the Nusselt number (a dimensionless number). There are also online calculators (http://www.heat-transfer-fluid.com/resources/heat-transfer-coefficient-calculator.php) available specifically for heat transfer fluid

applications.

Derivation of Convective heat transfer coefficient

An understanding of convection boundary layers is necessary to understanding convective heat transfer between a surface and a fluid flowing past it. A thermal boundary layer develops if the fluid free stream temperature and the surface temperatures differ. A temperature profile exists due to the energy exchange resulting from this temperature difference The heat transfer rate can then be written as,

And because heat transfer at the surface is by conduction,

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Rearranging,

Making it dimensionless by multiplying by representative length L,

The right hand side is now the ratio of the temperature gradient at the surface to the reference temperature gradient. While the left hand side is similar to the Biot modulus.This becomes the ratio of conductive thermal resistance to the convective thermal resistance of the fluid, otherwise know as the Nusselt number, Nu.

Convective heat transfer Correlations

Although convective heat transfer can be derived analytically through dimensional analysis, exact analysis of the boundary layer, approximate integral analysis of the boundary layer and analogies between energy and

momentum transfer, these analytic approaches may not offer practical solutions to all problems when there are no mathematical models applicable. As such, many correlations were developed by various authors to estimate the convective heat transfer coefficient in various cases including natural convection, forced convection for internal flow and forced convection for external flow. These empirical correlations are presented for their particular geometry and flow conditions. As the fluid properties are temperature dependent, they are evaluated at the film temperature

T

_{f, which is the average of the surface}

T

_{s and the surrounding bulk temperature,} .

Natural convection

External flow, Vertical plane

Churchill and Chu correlation[1] for natural convection adjacent to vertical planes. NuL applies to all fluids for both laminar and turbulent flows. L is the characteristic length with respect to the direction of gravity.

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External flow, Vertical cylinders

For cylinders with their axes vertical, the expressions for plane surfaces can be used provided the curvature effect is not too significant. This represents the limit where boundary layer thickness is small relative to cylinder diameter D. The correlations for vertical plane walls can be used when

External flow, Horizontal plates

W.H. McAdams [2] suggested the following correlations. The induced buoyancy will be different depending upon whether the hot surface is facing up or down. For a hot surface facing up or a cold surface facing down,

For a hot surface facing down or a cold surface facing up,

The length is the ratio of the plate surface are to perimeter. If the plane surface is inclined at an angle θ, the equations for vertical plane by Churchill and Chu may be used for θ up to

₆₀

o. When boundary layer flow is laminar, the gravitational constant g is replaced with g cosθ for calculating the Ra in the equation for laminar flow

External flow, Horizontal cylinder

For cylinders of sufficient length and negligible end effects, Churchill and Chu has the following correlation for

10

− 5

< Ra

D

< 10

12

External flow, Spheres

For spheres, T. Yuge [3] has the following correlation. for Pr≃1 and

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Internal flow, Laminar flow

Sieder and Tate [4] has the following correlation for laminar flow in tubes where D is the internal diameter, μ_b is the fluid viscosity at the bulk mean temperature, μ_w is the viscosity at the tube wall surface temperature.

Internal flow, Turbulent flow

The Dittus–Boelter correlation (1930) is a common and particularly simple correlation useful for many

applications. This correlation is applicable when forced convection is the only mode of heat transfer; i.e., there is no boiling, condensation, significant radiation, etc. The accuracy of this correlation is anticipated to be ±15%. For a liquid flowing in a straight circular pipe with a Reynolds number between 10 000 and 120 000 (in the turbulent pipe flow range), when the liquid's Prandtl number is between 0.7 and 120, for a location far from the pipe entrance (more than 10 pipe diameters; more than 50 diameters according to many authors[5]) or other flow disturbances, and when the pipe surface is hydraulically smooth, the heat transfer coefficient between the bulk of the fluid and the pipe surface can be expressed as:

where

k

_{w - thermal conductivity of the liquid (i.e.water)}

D

_{H -}

D

_{i - Hydraulic diameter}

Nu - Nusselt number

(Dittus-Boelter correlation) Pr - Prandtl number

Re - Reynolds number

n = 0.4 for heating (wall hotter than the bulk fluid) and 0.33 for cooling (wall cooler than the bulk fluid).[6] The fluid properties necessary for the application of this equation are evaluated at the bulk temperature thus avoiding iteration

Forced convection, External flow

In analyzing the heat transfer associated with the flow past the exterior surface of a solid, the situation is complicated by phenomena such as boundary layer separation. Various authors have correlated charts and graphs for different geometries and flow conditions. For Flow parallel to a Plane Surface, where x is the

distance from the edge and L is the height of the boundary layer, a mean Nusselt number can be calculated using the Colburn analogy.[7]

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There exist simple fluid-specific correlations for heat transfer coefficient in boiling. The Thom correlation is for flow boiling of water (subcooled or saturated at pressures up to about 20 MPa) under conditions where the nucleate boiling contribution predominates over forced convection. This correlation is useful for rough estimation of expected temperature difference given the heat flux:[8]

where:

ΔT

_{sat is the wall temperature elevation above the saturation temperature, K} q is the heat flux, MW/m2

P is the pressure of water, MPa

Note that this empirical correlation is specific to the units given.

Heat transfer coefficient of pipe wall

The resistance to the flow of heat by the material of pipe wall can be expressed as a "heat transfer coefficient of the pipe wall". However, one needs to select if the heat flux is based on the pipe inner or the outer diameter. Selecting to base the heat flux on the pipe inner diameter, and assuming that the pipe wall thickness is small in comparison with the pipe inner diameter, then the heat transfer coefficient for the pipe wall can be calculated as if the wall were not curved:

where k is the effective thermal conductivity of the wall material and x is the wall thickness.

If the above assumption does not hold, then the wall heat transfer coefficient can be calculated using the following expression:

where d_i and d_o are the inner and outer diameters of the pipe, respectively.

The thermal conductivity of the tube material usually depends on temperature; the mean thermal conductivity is often used.

Combining heat transfer coefficients

For two or more heat transfer processes acting in parallel, heat transfer coefficients simply add:

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For example, consider a pipe with a fluid flowing inside. The rate of heat transfer between the bulk of the fluid inside the pipe and the pipe external surface is:

where

q = heat transfer rate (W)

h = heat transfer coefficient (W/(m2·K)) t = wall thickness (m)

k = wall thermal conductivity (W/m·K) A = area (m2)

ΔT

= difference in temperature.

Overall heat transfer coefficient

The overall heat transfer coefficient

U

is a measure of the overall ability of a series of conductive and convective barriers to transfer heat. It is commonly applied to the calculation of heat transfer in heat exchangers, but can be applied equally well to other problems.

For the case of a heat exchanger,

_U

can be used to determine the total heat transfer between the two streams in the heat exchanger by the following relationship:

q = UAΔT

_LM

where

q

= heat transfer rate (W)

U

= overall heat transfer coefficient (W/(m²·K))

A

= heat transfer surface area (m2)

ΔT

LM = log mean temperature difference (K)

The overall heat transfer coefficient takes into account the individual heat transfer coefficients of each stream and the resistance of the pipe material. It can be calculated as the reciprocal of the sum of a series of thermal

resistances (but more complex relationships exist, for example when heat transfer takes place by different routes in parallel):

where

R = Resistance(s) to heat flow in pipe wall (K/W) Other parameters are as above.[10]

The heat transfer coefficient is the heat transferred per unit area per kelvin. Thus area is included in the equation as it represents the area over which the transfer of heat takes place. The areas for each flow will be different as they represent the contact area for each fluid side.

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The thermal resistance due to the pipe wall is calculated by the following relationship:

where

x = the wall thickness (m)

k = the thermal conductivity of the material (W/(m·K)) A = the total area of the heat exchanger (m2)

This represents the heat transfer by conduction in the pipe.

The thermal conductivity is a characteristic of the particular material. Values of thermal conductivities for various materials are listed in the list of thermal conductivities.

As mentioned earlier in the article the convection heat transfer coefficient for each stream depends on the type of fluid, flow properties and temperature properties.

Some typical heat transfer coefficients include: Air - h = 10 to 100 W/(m2K)

Water - h = 500 to 10,000 W/(m2K)

Thermal resistance due to fouling deposits

Surface coatings can build on heat transfer surfaces during heat exchanger operation due to fouling. These add extra thermal resistance to the wall and may noticeably decrease the overall heat transfer coefficient and thus performance. (Fouling can also cause other problems.)

The additional thermal resistance due to fouling can be found by comparing the overall heat transfer coefficient determined from laboratory readings with calculations based on theoretical correlations. They can also be evaluated from the development of the overall heat transfer coefficient with time (assuming the heat exchanger operates under otherwise identical conditions). This is commonly applied in practice, e.g.[11] The following relationship is often used:

=

where

U

exp = overall heat transfer coefficient based on experimental data for the heat exchanger in the "fouled" state,

U

_{pre = overall heat transfer coefficient based on calculated or measured ("clean heat exchanger") data,}

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References

1. ^ James R. Welty; Charles E. Wicks; Robert E. Wilson; Gregory L. Rorrer., "Fundamentals of Momentum, Heat and Mass transfer" 5th edition, John Wiley and Sons

5. ^ S.S. Kutateladze and V.M. Borishanskii, A Concise Encyclopedia of Heat Transfer, Pergamon Press, 1966. 6. ^ F.Kreith (editor), "The CRC Handbook of Thermal Engineering", CRC Press, 2000.

8. ^ W.Rohsenow, J.Hartnet, Y.Cho, "Handbook of Heat Transfer", 3rd edition, McGraw-Hill, 1998.

9. ^ This relationship is similar to the harmonic mean; however, note that it is not multiplied with the number n of terms.

10. ^ Coulson and Richardson, "Chemical Engineering", Volume 1,Elsevier, 2000

11. ^ Turner C.W.; Klimas S.J.; Bbrideau M.G., "Thermal resistance of steam-generator tube deposits under single-phase forced convection and flow-boiling heat transfer", Canadian Journal of Chemical Engineering, 2000, vol. 78, No 1, pp. 53-60

External links

Overall Heat Transfer Coefficients (http://www.cheresources.com/uexchangers.shtml) Correlations for Convective Heat Transfer (http://www.cheresources.com/convection.shtml) Heat Transfer fundamentals (http://www.hrs-heatexchangers.com/en/resources/heat-transfer-fundamentals-01-05.aspx)

Heat Transfer Coefficient Calculator (http://www.heat-transfer-fluid.com/resources/heat-transfer-coefficient-calculator.php)

Retrieved from "http://en.wikipedia.org/wiki/Heat_transfer_coefficient" Categories: Convection | Heat transfer | Heat conduction

This page was last modified on 24 June 2011 at 11:49.

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Heat Transfer Coefficient

Heat transfer coefficient

ΔT

Contents

Derivation of Convective heat transfer coefficient

Convective heat transfer Correlations

T

T

Natural convection

₆₀

10

< Ra

< 10

k

D

D

ΔT

Heat transfer coefficient of pipe wall

Combining heat transfer coefficients

ΔT

Overall heat transfer coefficient

U

_U

q = UAΔT

q

U

A

ΔT

Thermal resistance due to fouling deposits

U

U

See also

References

External links