Determination of Critical Heat Flux during Boiling Heat Transfer

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BACKGROUND

The heat flux or thermal flux gives the amount of heat energy transfer through a given surface per unit time. It is a vector quantity while the heat rate is the scalar one. This property is studied at an infinitesimal small surface size. Critical heat flux can be defined as the point in the boiling curve where the nucleate film boiling converts into film boiling causing the continuous separation of vapour film through the heater and the liquid. At this point, the vapour generated through nucleate boiling prevents liquid from reaching and wetting the surface due to its bigger size.¹

The boiling curve is divided into three different sections:

1. Natural Convection region
   
2. Nucleate boiling region and,
   
3. Film boiling region
   

Natural convection region originates at a low-temperature difference of about 10^oC or less. In this region, heat transfer causes the liquid to get superheated which rises to the free liquid surface by natural convection. With the increase in temperature difference between the surface (T_s) and wetted region (T_w), nucleate boiling originates. This region is separated into two different parts. In the first one, very few amounts of bubbles are formed which further condenses into the liquid and do not reach the free surface. While the second region shows higher tendency of bubble formation all over the surface which condenses and rise all the way to the free surface. In the film boiling region, the rate of bubble formation becomes very high, and they start to coalesce and cover the surface with vapour film. The film rises with the unstable (Half of the region is covered with vapour film and another half with the nucleate film.) to the stable (completely covered with vapour film) region.²

The main purpose of this experiment is to determine the critical heat flux during the boiling heat transfer process.

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REQUIREMENTS

Materials:   Thermometer (0-110^o C) – 1 piece

Bulb (100 W) – 1 piece

Glass Beaker – 1 piece

Heater Coil R-1 (1 KW Nichrome wire)

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PROCEDURE

First of all, add a sufficient amount of distilled water in a container and then check if both the heaters are completely submerged or not. Now, make the proper electrical connections by using heater coil R-1 and the test heater wire across the studs. After the proper connection, switch ON the heater R-1 until required bulk temperature of the water is achieved (50^oC, 60^oC, or 70^oC). After switching off heater R-1, gradually increase the voltage by changing the variac position across the test heater. Note down the voltage and current at the point where the wire breaks.

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CONCLUSION

The process of determining the critical heat flux during boiling heat transfer has been discussed. Critical heat flux describes the thermal limit of a phenomenon where a phase change occurs during the heating process.

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REFERENCES

1. J. K. K. K. SM You, “Effect of nanoparticles on critical heat flux of water in pool boiling heat transfer,” Journal of Applied Physics, vol. 83, no. 16, pp. 3374-3376, 2003.
   
2. Y Katto, “Critical Heat Flux,” International Journal of Multiphase Flow, vol. 20, pp. 53-90, 1994.