The heat transfer coefficient is used as a fudge factor in calculating heat transfer in thermodynamics. The heat transfer coefficent is often calculated from the Nusselt number (a dimensionless number). Below is an example where it is used to find the heat lost from a hot tube to the surrounding area. Fudge factor is a margin over and above the required resources such as time, capital, human capital, cost required for a certain project. ... Heat transfer is the study of the energy transfer via either conduction, convection, or radiation. ... Thermodynamics (from the Greek thermos meaning heat and dynamis meaning power) is a branch of physics that studies the effects of changes in temperature, pressure, and volume on physical systems at the macroscopic scale by analyzing the collective motion of their particles using statistics. ... The Nusselt number is a dimensionless number which measures the enhancement of heat transfer from a surface which occurs in a real situation, compared to the heat transfer that would be measured if only conduction could occur. ... In dimensional analysis, a dimensionless number (or more precisely, a number with the dimensions of 1) is a pure number without any physical units. ...

where

Q = power input or heat lost

h = overall heat transfer coefficient

A = outside surface area of tubing

ΔT = difference in temperature between tubing surface and surrounding area

There are different heat transfer relations for different liquids, flow regimes, and thermodynamic conditions. A common example pertinent to many of the necessary power plant efficiency and thermal hydraulic calculations is the Dittus-Boelter heat transfer corelation, valid for water in a circular pipe with Reynolds numbers between 100 000 and 120 000 and Prandtl numbers between 0.7 and 120. An example is shown below where it is used to calculate the heat transfer from a tubing wall to water. Thermodynamics (Greek: thermos = heat and dynamic = change) is the physics of energy, heat, work, entropy and the spontaneity of processes. ... Efficiency is the capability of acting or producing effectively with a minimum amount or quantity of waste, expense, or unnecessary effort. ... Thermal hydraulics is the study of hydraulic flow in thermal systems. ... The Reynolds number is the ratio of inertial forces (vsρ) to viscous forces (μ/L) and is used for determining whether a flow will be laminar or turbulent. ... Prandtl Number is a dimensionless number approximating the ratio of momentum diffusivity and thermal diffusivity, It is defined as: where is the kinematic viscosity and α is the thermal diffusivity. ...

where

k_w = thermal conductivity of water

Nu = Nusselt number

= => Dittus-Boelter correlation for pipe flow

Pr = Prandtl number =

Re = Reynolds number =

D_H = hydraulic diameter

= mass flow rate

μ = water viscosity

C_p = heat capacity at constant pressure

A = cross-sectional area of flow

The heat transfer coefficient has SI units in watts per meter squared-kelvin. Often it can be estimated by dividing the thermal conductivity by a length scale. Heat transfer coefficients add inversely, like resistances. It can be thought of as a thermal resistance. Shown below is an addition of heat transfer coefficients where one is estimated as a thermal conductivity divided by a length scale. Mass flow rate is the movement of mass per time. ... Cover of brochure The International System of Units. ... In physics, thermal conductivity, k, is the intensive property of a material that indicates its ability to conduct heat. ... Thermal resistance has two different meanings: 1) the temperature difference across the structure when a unit of heat energy flows through it in unit time or 2) the temperature difference across a unit area of a material of unit thickness when a unit of heat energy flows through it in... In physics, thermal conductivity, k, is the intensive property of a material that indicates its ability to conduct heat. ...

where

Q = power input

h = heat transfer coefficient

t = tubing thickness

k = thermal conductivity of metal tube

A = cross-sectional area of flow

ΔT = difference in temperature between outer wall of tubing and sample water.

See also

• Overall Heat Transfer Coefficients
• Correlations for Convective Heat Transfer
• Heat
• Heat pump
• Thermal-hydraulics