Boiling heal transfer

B Calculate the heat flux and Its critical value associated with nucleate boiling, and examine the methods of boiling heal transfer enhancement,... 

Boiling is probably the most familiar form of heat transfer, yet it remains to be the least understood form. After hundreds of papers written on the subject, we still do not fully understand the process of bubble fonnation and we must still rely on cmpitical or semi-empirical relations to predict the rate of boiling heal transfer. 

Operation in the transition boiling regime is normally avoided in the design of heat transfer equipment, and thus no major attempt has been made to develop general correlations for boiling heal transfer in this regime. 

The pool boiling heal transfer relations given above apply to smooth surfaces Helow we discuss some methods to enhance heat transfer in pool boiling. 

The enhancement of boiling heal transfer in Freon-12 by a mechanically roughened surface, thermoexcel-E. 

Water is boiled at 120°C by a 0.6-m-long and 1,3-m diameter nickel-plated electric healing element maintained at 140°C. Determine (o) the boiling heal transfer coefficient,... 

Gorenflo, D., 1984, Behaltersieden, VDI-Warmeatlas, 4 Aufl., Abschnitt Ha VDI-Verlag, Dusseldorf. (2) Gorenflo, D., V. Knabe, and V. Bieling, 1986, Bubble Density on Surfaces with Nucleate Boiling—Its Influence on Heat Transfer and Burnout Heat Flux at Elevated Saturation Pressure, 8th Int. Heal Transfer Conf, Vol. 4, San Francisco. (2)... 

Monde, M., H. Kusuda, and H. Uehara, 1982, Critical Heat Flux during Natural Convective Boiling in Vertical Rectangular Channels Submerged in Saturated Liquid, Trans. ASME, J. Heal Transfer 704 300-303. (5)... 

The boiling temperature of nitrogen at atmospheric pressure at sea level (1 aim) is - 196 C. Therefore, nitrogen is commonly used in low lemperatnre scientific studies since the temperature ofliquid nitrogen in a tank open to the atmosphere remains constant at -196°C until the liquid nitrogen in the tank is depleted. Any heal transfer to the tank results ill the evaporation of some liquid iiiliogen. which has a heal of vaporization of 198 kJ/kg and a density of 810 kg/m at 1 atm. 

Consider a stainless slccl spoon (k = 15.1 V/m C) partially immersed in boiling water at 95°C in a kitchen at 25°C. The handle of the spoon has a cross section of 0.2 cm X 1.3 cm, and extends 18 cm in the air from the free surface of the water. If the heal transfer coefficient at the exposed surfaces of the spoon handle is 17 W/m - C, deieimine the temperature difference across the exposed surface of the spoon handle. Stale your assumptions. Ansr/er 69.8X... 

A 1.2-m-diameIer and 6-m-long cylindrical propane tank is initially filled with liquid propane whose density is 581 kg/m. The tank is exposed to the ambient air at 30°C, with a heal transfer coefficient of 25 W/m °C. Now a crack develops at the top of the tank and the pressure inside drops to 1 atm while the temperature drops to —42 C, which is the boiling temperature of propane at 1 atm. The heat of vaporisation of propane at 1 atm is 425 kJ/kg. The propane is slowly vaporised as a result of the heat transfer from the ambient air into the tank, and the propane vapor escapes the tank at -42 C through the crack. Assuming the propane lank to be at about the same... 

In the preceding chapters we considered forced and free convection heal transfer involving a single phase of a fluid. The analysis of such convectior processes involves tlie thermophysical properties p, p, k, and Cp of the fluid The analysis of boiling heat transfer involves these properties of the liquic (indicated by the subscript 0 or vapor (indicated by the subscript v) as well a the properties (the latent heat of vaporization) and cr (the surface tension) The hf represents the energy absorbed as a unit mass of liquid vaporize ... 

You may be tempted to simply add the convection and radiation heat transfers to determine the total heat transfer during film boiling. However, these two mechariisms of heat transfer adversely affect each other, causing the total heal transfer to be less than their sum. For example, the radiation heat transfer from the surface to the liquid enhances the rale of evaporation, and thus the thickness of the vapor film, which impedes convection heat transfer. For flfiimi Broitdey dclcnuincd that die relation... 

SOLUTION Water is boiled at 1 atm by a horizontal polished copper heating element. The rate of heal transfer to the water per unit length of the heater is to be-deteimined,... 

Tliese regimes are illustrated on the boiling curve. The rate of evaporaiion and the rate of heal transfer in nucleate boiling increase with increasing and reach a maximum at some... 

P. J. Berensen, Filin Boiling Heat Transfer for a Hoiironlal Surface." Journal of Heal Transfer 83 (1961), pp. 351-35. ... 

Water is to be boiled at atmospheric pressure in a mechanically polished steel pan placed on top of a heating unit, llie inner surface of the bottom of the pan is maintained at 1 lO C. If the diameter of the bottom of the pan is 30 cm, determine (a) the rale of heal transfer to the water and (b) the rate of evaporation. 

Water is boiled at 100°C by a spherical platinum heating element of diameter 15 cm and emissivity 0.10 immersed in the water. If the surface temperature of the heating element i,s 350°C, determine the rale of heal transfer from the heating clement to the water. 

Heal balance assumptions Operating temperature, 60"C operating pressure, 20 in. vacuum Note Boiling point of benzene at 20 in. of vacuum is 45.5 C) drying time required, 1 hr minimum over-all heat-transfer coefficient, 10 Btu/hr water (in jacket) in at SO C, out at 60 C. 

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