The pool boiling techniques are using in various industries like processing, thermal, refrigeration and air conditioning, production etc. The interest in enhanced heat transfer is closely tied to energy prices. In the 1980s and 1990s, energy prices were low due to oil and natural gas "bubbles". Recently, however, increased demand and inadequate supply or distributions have resulted in large increases in the price of energy. There is now an incentive to save energy, and enhanced heat transfer can be exploited to do so. Whereas in the previous two decades, enhancement was employed to reduce the size of equipment, thereby saving space, it is now applied to save energy costs. Due to the energy crisis problem, the aim is to reduce the energy required for phase change during the pool boiling. For increasing heat transfer rate the boiling curve must shift towards right. There are following techniques to increase the heat transfer rate in pool boiling. [8]
Nucleate pool boiling of pure water and water with cationic surfactant on the horizontally heated tube was examined. The motion of bubbles and temperature of heated surface were recorded by high speed video camera and an infrared radiometer.
1. Bubbles form in Habon-G solutions was very much smaller than those in water and surface becomes covered with them faster. It is known that reduced surface tension results in a decrease of energy required to create bubble and thus in more bubbles and smaller ones.
2. The histograms of thermal fields of heated wall measured by IR technique show the decrease in average temperature and increase in magnitude of standard deviation for surfactant solutions as compared with water. It means heat transfer coefficient in the surfactant solution increases and vaporization is more intensive compared with water boiling.
3. The heat transfer increases at low Habon-G surfactant concentrations, reaches maximum and decreases with further increase in concentrations.
Experimental Set up
The apparatus consists of a cylindrical glass container housing the test heater and the heating coil for the initial heating of the water. The heater coil is directly connected to the mains (Heater R1) and the test heater (Nichrome wire) is connected also to mains via a dimmerstat. An ammeter (range 0-10 Amp.) is connected in series while the voltmeter across it to read the current and voltage. Voltage selector switch is used to select the voltage range 50/100 Volts. These controls are placed inside the control panel.
The glass container is kept on a stand which is fixed on a wooden platform. 2.5 liter water is filled inside the glass container. The temperature of bulk water i.e. saturation temperature of water is measured using thermometer while Cr-Al k-type thermocouple is connected to nichrome heater wire to measure the temperature of wire using digital temperature indicator having least count 0.10C. The schematic arrangement of apparatus is shown in fig.1
Effects of surfactants on surface tension
The surface tension data at with and without surfactant in the water is taken from the literature. The surface tension of the water σ against temperature T is shown in the literature. The trend shows increase in temperature surface tension decreases. [14]
Hetsroni et al.[12] measured the surface tension data of water with Sodium Lauryl Sulphate surfactant using Sensa Dyne Surface Tensiometer System. The measurements of surface tension are carried out for different concentrations of SLS as a surfactant in pure water over a range of temperature 0-1200C. Figure 5.2 shows with increase in temperature surface tension decreases. They found that an increase in surfactant concentration up to 400 ppm leads to significant decrease in surface tension with increase in temperature, whereas the surface tension is almost independent of concentrations in the range 400 to 600 ppm. [12]
Fig. 2 Effects of temperature on surface tension of water
Fig. 3 Effects of temperature and concentration of Sodium lauryl sulphate on
surface tension
Surface tension data for different concentrations of Hydroxyethyl Cellulose is given in the Handbook of Pharmaceutical Excipients.[16] Figure 5.3 shows an increase in surfactant concentration up to 400 ppm leads to significant decrease in surface tension, whereas the surface tension is almost independent of concentrations in the range 400-600 ppm. But in all cases an increase in a liquid temperature leads to a decrease in the surface tension. [16]
Fig. 5 Effects of temperature and concentration of Hydroxyethyl cellulose on surface tension
Boiling Behavior
Study of pool boiling phenomena for water with and without surfactants i.e. the evolution of vapor bubbles in a boiling. The growth of bubbles is one of the parameters determining the intensity of the heat transfer from a heated surface. The growth of the bubble in the liquid containing surfactants is affected by a number of specific factors. The pool boiling experiments are carried out under atmospheric pressure. The phenomenon of foaming, often observed during boiling in the presence of surfactant SLS for all solutions. The foam formed on the surface of the solution and its height increased with the heat flux. In the present study, the height of the liquid phase over the heater is not less than 35 mm throughout all experiments. The bubble behavior is recorded at 24 frames per second by the video camera. The typical stages of bubble growth analyzed for this study are shown in figs. 5.4 - 5.10
Figure 5.4 (a-c) show typical pictures for deionized water boiling on the nichrome wire at heat fluxes 176.5, 287.4, 441.3 kW/ m2 respectively. A population of bubbles was observed in the vicinity of the heated wire. The bubble dynamics for water are seen to depend on heat flux, similar to well-known boiling visualization data. After the onset of nucleate boiling, the regime of single bubbles occurs close to the heated wall (Fig. 5.4a). As the heat flux increases, bubble coalescence take place (Fig. 6b). This phenomenon is more pronounced at heat flux 441.3 kW/m2 (Fig. 6c). For pure water, the average bubble size was observed to slightly increase with increasing heat flux. The bubbles have an irregular shape at all values of heat flux.
a)
b)
c)
Fig 6 Boiling of pure water for heat fluxes (kW/m2): a) 176.5; b) 287.4; c) 441.3
Figure 6 (a-c) shows boiling of the 400 ppm SLS added in pure water under the same conditions as that of water. The SLS additive reduces significantly the tendency of coalescence between vapor bubbles. Here too, there is a weak tendency toward increasing the average diameter as heat flux increases. In this case, the shape of bubbles is closer to spherical than for pure water. It presents cluster of small bubbles, which rise from the cavity. The bubbles are adjacent to each other and the cluster neck is not observed.
The comparison between pure water and the water with SLS are described using the photographic observation. Figure 5.6 shows the photographs for 100-600 ppm of SLS in water. It shows with increase in heat flux i.e. temperature, bubble size also increases and detaching time of the bubble is less as compared to water. Diameter of bubble in surfactant solution is smaller as compared to water. Bubbles form a blanket over the wire, which could not observed in water. The reason is due to addition of varying concentration of SLS in water, decreases the surface tension of solution and hence the forces acting on bubbles are smaller and detach it rapidly from the surface. Also surfactant activates the number of nucleation sites on the same wire. The approximate bubble velocity is measured using the software Windows movie maker and Adobe Photoshop 7.0. The time between detaching bubbles from wire and reaching the bubble up to free surface is measured in Windows movie maker and the distance between wire and free surface is measured in Adobe Photoshop 7.0. The same procedure is repeated in three times for every concentration and average velocity is calculated. The results show that the velocity
a)
b)
c)
Fig. 7 Boiling of 400 ppm SLS in water for heat fluxes (kW/m2): a) 176.5; b) 287.4; c) 441.3
Conclusions
The experimental results are summarized as follows:
Bubble action is seen to be extremely chaotic, with extensive coalescence during the rise for pure water.
The addition of small amount of anionic surfactant Sodium Lauryl Sulphate SLS and polymeric surfactant Hydroxyethyl Cellulose HEC-H in water makes the boiling behavior quite different from that of pure water. It might be that reduction in surface tension results in a decrease of energy required to create a bubble.
The bubbles formed in water with surfactant solutions are much smaller than pure water and they covered the surface of wire faster.
The boiling excess temperature ∆Texcess becomes smaller and the vapor bubbles are formed more easily. It might be due to presence of surfactant in water promotes activation of nucleation sites in a clustered mode.
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[2] T. Inoue, Y. Teruya & M. Monde. (2004), "Enhancement of Pool Boiling Heat Transfer in Water and Ethanol/Water Mixtures with Surface Active Agent", International Journal of Heat and Mass Transfer 4, pp. 5555-5563.
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[5] P. Kotchaphakdee and Michael C. Williams. (1970) "Enhancement of Nucleate Pool Boiling with Polymeric Additives", International Journal of Heat and Mass Transfer 13, pp. 835-848.
[6] Wade R. McGill, John S. Fitch, William R. Hamburgen & Van P. Carey. (1990), "Pool Boiling Enhancement Techniques for Water at Low Pressure", Western Research Laboratory USA, pp. 1-25.
[7] Erwin Orquiola & Yasunobu Fujita. (March, 2002) "Contact Angle Effects in Boiling Heat Transfer", Memories of the Faculty of Engineering, Kyushu University, Vol. 62, No.1, pp. 55-65.
[8] Arthur E Burgles (May 2003), "High Flux Processes Through Enhanced Heat Transfer", 5th International Conference on Boiling Heat Transfer, Montego Bay, Jamaica.
[9] Vijay K. Dhir (January 2006), "Mechanistic Prediction of Nucleate Boiling Heat Transfer - Achievable or Hopeless Task?", ASME Journal of Heat Transfer, Vol-128, pp. 1-12.
[10] E. Laurien, T.Giese, D.Saptoadi & J.Schutz. (2003), "Multidimensional Modeling and Simulation of Subcooled Pool Boiling", Institute for Nuclear Technology and Energy Systems (IKE), University of Stuttgart, Germany.
[11] E. Laurien and D. Saptoadi. (June 2001), "On The Fundamental Two- Fluid Equations to Model Three-Dimensional Bubbly Flow", 4th International Conference on Multiphase Flow, ICMF'01, New Orleans.
[12] G. Hetsroni, J.L. Zakin, Z. Lin, A. Mosyak, E.A. Pancallo & R. Rozenblit. (2001), "The effect of surfactants on bubble growth, wall thermal patterns and heat transfer in pool boiling", International Journal of Heat and Mass Transfer 44, pp. 485-497.
