Convective Heat Transfer Coefficient Meaning, Calculation Examples
Convective heat transfer coefficient h (or hc), is a measure of the thermal resistance of a thin layer of relatively immobile fluid, which is sandwiched between a solid heat exchanger and a fluid medium. This immobile fluid layer is often called the boundary layer [3].
It may also be described as a measure of the rate at which thermal energy is transferred into a convective medium from a heat source to a layer of fluid.
The understanding and estimation of convective heat transfer coefficient are very important when studying convection in any fluid system, as a means by which details of the convection energy transfer mechanism can be fully revealed.
Convective heat transfer coefficient can be distinguished into natural and forced types; where the natural convective heat transfer coefficient is that which occurs in free convection systems, and the latter occurs in artificially-driven convection systems.
This article discusses convective heat transfer coefficient meaning, calculation, and examples, as outlined below;
-How to Calculate Convective Heat Transfer Coefficient
-Convective Heat Transfer Coefficient of Materials
-Factors that Affect the Coefficient of Convective Heat Transfer
How to Calculate Convective Heat Transfer Coefficient
As earlier stated, convective heat transfer coefficient measures the resistance to heat transfer from a solid surface to a stagnant fluid layer, and how this affects the rate of convection in the bulk fluid medium [1].
Convective heat transfer coefficient can be calculated by dividing the rate of heat transfer in a convective medium, by the product of heat transfer area and temperature gradient.
The convective heat transfer formula is given in the equation below;
hc = q/A dT —(1)
where;
hc = convective heat transfer coefficient of the process (W/(m2K), kcal/(h m2 oC), or Btu/(ft2 h oF))
q = rate of heat transfer (W or Btu/hr)
A = surface area of heat transfer (m2 or ft2)
dT = temperature gradient between the heat transfer surface and the bulk fluid (oC, K, or F)
The equation can be expanded into the following form;
hc = q/A (Th -Ta) —(2)
where Th and Ta represent the temperatures of the hot surface in contact with the fluid, and that of the bulk fluid itself.
The formula for convective heat transfer coefficient is derived from the formula for heat transfer rate, which is;
q = hc A dT —(3)
Convective heat transfer coefficient unit could be either kcal/(h m2 oC), W/(m2K), or Btu/(ft2 h oF). These units are related to each other as shown below;
1 kcal/(h m2 oC) = 1.163 W/(m2K) = 0.205 Btu/(ft2 h oF) —(4)
1 W/(m2K) = 0.85984 kcal/(h m2 oC) = 0.1761 Btu/(ft2 h oF) —(5)
1 Btu/(ft2 h oF) = 5.678 W/(m2 K) = 4.882 kcal/(h m2 oC) —(6)
Calculating the convective heat transfer coefficient is possible for a thermodynamic system in which thermal energy is being transferred from the surface of a hot solid, to a layer of fluid that is in contact with this surface; so that heat is introduced into the system by conduction, before it is subsequently circulated in the fluid medium by convection.
One of such thermodynamic systems is a conductive water vessel (like a kettle, pot or tank) into which heat is introduced from the external environment by conduction through the conductive surface, and circulated as the fluid masses gain heat and experience changes in density.
Real-life examples of systems that meet all criteria for calculation of convective heat transfer coefficient include water-filled (conductive) vessel on a burner, and solar water heater exposed to radiation.
Convective Heat Transfer Coefficient of Materials
For various materials, the coefficient of convection varies with specific characteristics like composition and phase of occurrence (liquid, gaseous, solid).
In solid state, convection heat transfer coefficient of metals can be measured using a thermocouple setup that usually includes these solids as thin wires in a convective system [2].
Otherwise, metals like copper and aluminum can be measured for their convective coefficients when these metals are in fluid state, such as molten/liquid metal form. For metals like mercury that occur as liquid at room temperature, measuring convective coefficient can be easily done using similar methods as those for other conductive liquids.
Also, the value of convective heat transfer coefficient varies with the type of convection involved, which could be forced or natural/free convection.
The table of convective heat transfer coefficients below provides values for multiple materials.
These include the convective heat transfer coefficient of air, water, aluminum, copper and steel [4].
Material | Convection Type/Phase | Convective Heat Transfer Coefficient |
Air | Free Convection | 0.5 – 1000 (W/(m2K)) |
Forced Convection | 10 – 1000 (W/(m2K)) | |
Water | Free Convection | 50 – 3000 (W/(m2K)) |
Forced Convection | 50 – 10000 (W/(m2K)) | |
Aluminum | Solid State (Thermocouple) | 61.5 (W/(m2K)) |
Liquid State | 5000 – 40000 (W/(m2K)) | |
Copper | Solid State (Thermocouple) | 62.5 (W/(m2K)) |
Liquid State | 5000 – 40000 (W/(m2K)) | |
Steel | Solid State (Thermocouple) | 57.5 (W/(m2K)) |
Liquid State | 5000 – 40000 (W/(m2K)) |
Other values of convective heat transfer coefficient are given below;
Material | Convection Type/Phase | Convective Heat Transfer Coefficient |
Dry Vapor | Free Convection | 0.5 – 1000 (W/(m2K)) |
Forced Convection | 10 – 1000 (W/(m2K)) | |
Gases | Free Convection | 0.5 – 1000 (W/(m2K)) |
Forced Convection | 10 – 1000 (W/(m2K)) | |
Condensing Water Vapor | Gaseous Phase | 5.0– 100.0 (W/(m2K)) |
Boiling Water | Solid State (Thermocouple) | 3.0 – 100.0 (W/(m2K)) |
Liquid State | 5000 – 40000 (W/(m2K)) | |
Liquid Metals | ||
Forced Convection | 5000 – 40000 (W/(m2K)) | W/(m2K)) |
The convective heat transfer coefficient of materials tends to vary with change in temperature of the materials. Generally, the variation of convective coefficient with temperature is one of inverse proportionality.
This means, for example, that the convective heat transfer coefficient of air at different temperatures, will decrease per unit increase in temperature; so that convection coefficient of air at 25 °C will be lower than that at 100 °C, and higher than the value at 10°C.
Factors that Affect the Coefficient of Convective Heat Transfer
Convective heat transfer depends on various factors like;
1). Material composition
2). Phase (solid, liquid, gaseous) of materials involved
3). Fluid velocity
4). Thermophysical properties like conductivity
5). Fluid flow trend/hydrodynamic characteristics (turbulent, laminar)
6). Instantaneous temperature conditions
7). Surface area of materials in contact
Conclusion
Convective heat transfer coefficient is a variable that expresses the rate of heat transfer in a convective medium, as a function of fluid thermal resistance.
The formula for convective heat transfer is as follows;
hc = q/A dT
or
hc = q/A (Th -Ta)
Convective heat transfer coefficient is measured in unit of kcal/(h m2 oC), W/(m2K), or Btu/(ft2 h oF).
Convective heat transfer depends on; material composition, phase, fluid velocity, thermophysical attributes, flow trend, instantaneous temperature, and surface area of materials in contact.
References
1). Awbi, H. B. (1998). “Calculation of convective heat transfer coefficients of room surfaces for natural convection.” Energy and Buildings 28(2):219-227. Available at: https://doi.org/10.1016/S0378-7788(98)00022-X. (Accessed 2 April 2023).
2). Khalifa, A. J.; Al Mousawi, I. R. A. (2016). “Comparison of Heat Transfer Coefficients in Free and Forced Convection using Circular Annular Finned Tubes.” Available at: https://www.researchgate.net/publication/336413195_Comparison_of_Heat_Transfer_Coefficients_in_Free_and_Forced_Convection_using_Circular_Annular_Finned_Tubes. (Accessed 2 April 2023).
3). Wan, J. (2021). “The Heat Transfer Coefficient Predictions in Engineering Applications.” Journal of Physics Conference Series 2108(1):012022. Available at: https://doi.org/10.1088/1742-6596/2108/1/012022. (Accessed 2 April 2023).
4). Yener, T.; Yener, Ş, C.; Mutlu, R. (2019). “CONVECTION COEFFICIENT ESTIMATION OF STILL AIR USING AN INFRARED THERMOMETER AND CURVE-FITTING.” Journal of Engineering Technology and Applied Sciences. Available at: https://doi.org/10.30931/jetas.598862. (Accessed 2 April 2023).