Heat Transfer Phenomena in Oxyacetylene Gas Welding

Heat Transfer Phenomena in Heat Gun – Effect of Melting
Heat gun is actually a one of the common power tools which usually can be found in autoshop. This particular of tool looks alike to hair dryer (that normally being used in house to drying wet hair). Heat gun is actually pretty handy tool as it can be used to desoldering electronic components, shrink warping and heat shrink tubing, bending PVC pipe and tube, window tinting, vinyl and leather repair, as well as many others unconventional creative manipulation of using heat gun. Figure 1 and Figure 2 shows some uses of heat gun in action. One of big advantage of using heat gun is it is able to operate flamelessly since flame can damage the work surface and potentially to cause fire disaster. Normally the hot air from this tool able to reach up from 100°C to 550°C. This make heat gun very useful. Figure 1: PVC pipe bending. [1] Figure 2: Loosen the new bike seat before cover installation. [2]
How does this tool works? The source of energy used in heat gun is generally comes from electric energy. The electrically heated elements in this tool convert electric energy into useable heat energy. Next, a blower or fan is used to make flow of air. The fresh air then make contact with hot heat element is absorbing the heat causing heat transfer phenomena to occur. This is forced convection heat transfer modes as it involve fluid (in this case the air) in forced motion. Hot stream of air is then directed to work surface by using nozzle which comes in various size and shape. The second forced convection heat transfer modes the takes place as hot stream of air make contact with work surface which usually involve material sub-melting. Even though every matter producing radiation mode of heat transfer, in this case we only focus on forced convection heat transfer mode.
Chi and Yin [3] in their paper state that the theory behind effect of melting on forced convection heat transfer can be analysed using three theories; the boundary layer theory, film theory, and penetration theory. The boundary layer theory can be imagine as fluid flowing laminarly over a solid plate. For a simplified the analysis, a steady-state two-dimensional case can take as generalise assumption. The continuity equation, equation of motion, and equation of energy can be written as follow respectively.

(1)

(2)

(3)
Where u is velocity in x-axis, v is velocity in y-axis, μ is dynamic viscosity of fluid, ρ is density of fluid, and T is temperature. The result of melting and its magnitude makes possible for existence of interfacial velocity (is the velocity of the front across which the phases are different). The interfacial velocity is related to heat flux transferred from the fluid to plate as shown in Equation 4.

(4)
Where k is thermal conductivity of fluid, ΔHm is enthalpy change due to melting. On top of all, the ratio of heat transfer coefficient with melting and without melting, θT for boundary layer theory, film theory, and the penetration theory, the ratio is denoted as Equation 5, 6, and 7 respectively.

(5)

(6)

(7)

(8)

(9)
Where hm is heat transfer coefficient with melting, hwm is heat transfer coefficient without melting, Pr is Prandtl number, Cp is heat capacity of fluid at constant pressure, T∞ is temperature of fluid at infinity, T0 is temperature at surface. The value for θT as function of A are as shown in Figure 3. Even the result different with each other, the same general graph patent can be seen. This may suggest that the melting preventing the heat transfer rate. Generally for the same magnitude of θT, the value of A is slightly higher for film theory compared to boundary layer theory and penetration theory.

Figure 3: Relationship between parameter A and θT. [3]
The relationship between rates of heat transfer with performance of heat gun is very importance. Without heat transfer, this tool cannot even work as this tool’s goal is to transfer heat produce by heating element to the work piece via stream of hot air in forced convection mode. Increasing the blower power will likely to improve the performance of heat gun.
REFERENCE
[1] Wagner-group (2014). [Graphical illustration of PVC bending using heat gun].
HT3500 Heat gun. Retrieved from http://www.wagnerspraytech.com/portal/ht3500_en_spray,363646,358970 .html
[2] Burns, J. (2011). [Graphical illustration of loosen the new bike seat before cover installation]. Kawasaki KDX 200 Refresh, Part 3: Seat Cover, Exhaust Pipe Repair.
Retrieved from http://www.off-road.com/dirtbike/kawasaki-kdx-200-refresh-part-3-seat- cover-exhaust-pipe-repair-53302.html [3] Chi Tien and Yin-Chao Yen (1965). The effect of melting on forced convection heat transfer. Journal of Applied Meteorology, Vol. 4. 523-527.