FIELD OF THE INVENTION
The present invention generally relates to a method for improvement of mixed convective heat transfer performance of a heat reservoir with the use of ultrasonic wave on a heat conducting fluid. More particularly, the invention relates to a system for mixed convective heat transfer in a heat reservoir augmented by application of ultrasonic waves for increase of localized heat transfer rate.
BACKGROUND OF THE INVENTION
For a long period, researchers have attempted to enhance the heat transfer either in a flowing circuit or in a stagnant reservoir. Mainly, researchers have taken the avenues of enhancing the heat transfer by modifying the surface of the heat transfer area or by enlarging the surface area. Apart from this, last decade has witnessed a significant rise in the research work of nanofluids for use as heat enhancing fluid. Addition of fin on heat dissipating surface is another mechanism for heat transfer enhancement by means of increase of surface area.
In essence, either the modification of the conducting medium or fluid has been attempted in the past and the research community is continuing its pursuit for the same. Also, common in the research investigation were the modification of the flowing circuit particularly, the shape of the circuit. Surface waviness or inducing roughness has been implemented to impart enhancement of the heat transfer. One of the external factors to modify the flow field for heat transfer enhancement is ultrasonic wave.

The measurement of the heat transfer enhancement is signified by the measured values of heat transfer coefficient and also by the thermal conductivity. Sound generated above the human hearing range (typically 20 kHz) is called ultrasound. In general, "Ultrasonic vibrations" refers to vibrations of sound waves at frequencies above the frequencies of audios sound with a range of frequency of 30 to 20,000 Hz. However, common frequency range normally employed for ultrasonic nondestructive testing and thickness gauging is 100 kHz to MHz. Due to the short wavelength of ultrasound, it can be cased to reflect from very small surfaces such as the defects inside materials, the property which makes ultrasound technology appropriate for nondestructive testing of materials.
Ultrasound (i.e., mechanical waves at a frequency above the threshold of human hearing) can be divided into three frequency ranges:
> Power ultrasound (16 - 100 kHz)
> High frequency ultrasound (100 kHz - 1 MHz)
> Diagnostic ultrasound (1-10 MHz)
Ultrasound can induce some special mechanical effects when it transmits through medium, typically like cavitation in liquid environment and acoustic streaming in liquid or gas environment. Cavitation refers to the formation and subsequent dynamic behavior of vapor bubbles in liquids. It occurs when the acoustic intensity is high enough and the sound waves are coupled to the liquid surface, which results in the propagation of alternating regions of compression and expansion, and thus in the formation of micro-size vapor bubbles (Suslick, 1998). Small bubbles are generated when a liquid is subjected to ultrasonic vibrations.

Cavitation refers to the formation and subsequent dynamic behavior of vapour bubbles in liquids formed when the acoustic intensity is large enough and the sound waves are coupled to the liquid surface. This gives rise to the propagation of alternating regions of compression and expansion, and consequently the formation of micro-size vapor bubbles (Suslick, 1998). In fact, the generation and collapse of so formed small bubbles, known as acoustic cavitation, can be expected to have a pronounced effect upon any boundary layer vis-a-vis the surface in which they are found. A steady circulation is generated near the surfaces of obstacles and vibrating elements, and near bounding walls in a high-intensity sound field when subjected to acoustic streaming (Lee and Wang[12] and this phenomenon makes the ultrasonic for suitable application in the field of the heat transfer enhancement under different circumstances, such as natural convection heat transfer. During the imposition of ultrasonic wave, the oscillation of the bubble (within in a critical size range) wall matches that of the applied frequency of the sound waves and causes the implosion of the bubbles during a single compression cycle (Moholkar, Rekveld, and Warmoeskerken, 2000[16]). The collapse of the bubbles leading to the phenomenon of cavitation is the most important phenomenon during high power ultrasonics when the bubble reaches a temperature of 5000 K and pressures of up to 1000 atmospheres. This high temperature and pressure in turn produces very high shear energy waves and turbulence in the cavitation zone (Suslick, 1988[20]); Laborde, Bouyer, Caltagirone, and Gerard, 1998[11]).
In general, a transducer, a device to convert one form of energy to another, is used for imparting ultrasonic wave into a liquid medium. An ultrasonic transducer

converts electrical energy to mechanical energy, in the form of sound, and vice versa.
Ultrasound on a continuum fluid induces the imposition of an acoustic pressure (Pa) in addition to the hydrostatic pressure that is already present on the medium. Mathematically, the acoustic pressure (Pa) generated by ultrasound can be represented by a sinusoidal wave dependent on time (t), frequency (f), and the maximum pressure amplitude of the wave [18]
Pa = Pa max sin(2ƒt) [1]
Where the maximum pressure amplitude of the wave (Pa,max) is directly proportional to the power input of the transducer. The pressure wave induces motion and mixing within the fluid, at low intensity (amplitude) is known as acoustic streaming (Leighton[13] while, at higher intensities, the local pressure in the expansion phase of the cycle falls below the vapor pressure of the liquid, causing tiny bubbles to grow (created from existing gas nuclei within the fluid).
The state of art of using ultrasound wave for enhancement of heat transfer known to the inventors can be summarized as under:
[1] Bonekamp, S., Bier, K., 1997, influence of Ultrasound on Pool Boiling Heat Transfer to Mixtures of Refrigerants R23 and R134A. International Journal of Refrigeration, vol.20, no. 6:p606-615

[2] Bergles, A. E., 1969, Survey and evaluation of techniques to augment convective heat and mass transfer, Progress in Heat and Mass Transfer, vol. 1, pp.331-424,
[3] Bergles, A.E., Newell, P.H. 1965., The influence of ultrasonic vibrations on heat transfer to water flowing in annuli, International Journal of Heat and Mass Transfer, vol. 8, no. pp. 1273-1280.
[4] Fand, R.M., 1965, The influence of Acoustic Vibrations on Heat Transfer by Natural Convection from a Horizontal Cylinder to Water. Journal of Heat Transfer, vol. 87, no, 2: p309-310.
[5] Fand, R.M., Kave, J., 1960, Acoustic streaming near a heated cylinder, Journal of the Acoustical Society of America, vol. 32, pp. 579-584
[6] Hoyle, B.S., Luke, S.P., 1994, Ultrasound in the process industries, Engineering Science and Education Journal, vol. 3, no. 3, pp. 119-122.
[7] Hyun, S., Lee, D.R, Loh B.G., 2005, Investigation of Convective Heat Transfer Augmentation Using Acoustic Streaming Generated by Ultrasonic Vibrations. International Journal of Heat and Mass Transfer, vol. 48, no. 6: p703-718
[8] Kim, H.Y., Kim, Y.G., Kang B.H., 2004, Enhancement of Natural Convection and Pool Boiling Heat Transfer via Ultrasonic Vibration. International Journal of Heat and Mass Transfer, vol. 47, no. 11: p2831-2840.

[9] Larson, M.B., 1961, A study of the effects of ultrasonic vibrations on convective heat transfer in liquids, Ph. D. dissertation, Stanford University, Stanford, Calif, USA, Mic 61-1235.
[10] Larson, M.B., London, A.L., 1962, A Study of the Effects of Ultrasonic Vibrations on Convection Heat Transfer to Liquids, ASME Paper No. 62-HT-44.
[11] Laborde, J.-L., Hita, Caltagirone, A., Gerard, J.-P. A., 2000, Fluid dynamics phenomena induced by power ultrasounds, Ultrasonics, vol. 38, pp. 297-300.
[12] Lee, C.P., Wang T.G., 1990, Outer Acoustic Streaming. Journal of the Acoustical Society of America, vol. 88, no. 5: pp2367-2375
[13] Leighton, T.G. (1994). The Acoustic Bubble, Academic Press, San Diego.
[14] Li, K.W., Parker, J.D., 1967, Acoustical Effects on Free Convective Heat Transfer from a Horizontal Wire. Journal of Heat Transfer, vol. 89, no. 2: p277-278.
[15] lida, Y., Tsutsui, K., 1992, Effects of Ultrasonic Waves on Natural Convection, Nucleate Boiling and Film Boiling Heat Transfer from Wire to a Saturated Liquid. Experimental Thermal and Fluid Science, vol.5, no.l: pl08-115 [16] Moholkar, V.S., Rekveld, S., and Warmoeskerken, M.M.C.G. (2000). Modeling of the acoustic pressure fields and the distribution of the cavitation phenomena in a dual frequency sonic processor. Ultrasoncis, 38,666-670.

[17] Muthukumaran, S., Kentish, S.E., Stevens, G.W., and Ashokkumar, M. (2006). Application of Ultrasound in Membrane Separation processes: A Review. Rev. Chem. Eng., 22, 155-194.
[18] Nomura, S., Yamamoto, A., Murakami, K., 2002, Ultrasonic Heat Transfer Enhancement Using a Horn-type Transducer. Japanese Journal of Applied Physics, vol. 41, no. 5B: p3217-3222
[19] Park, K.A., Bergles, A.E., 2004, Ultrasound Enhancement of Saturated and Subcooled Pool Boiling. International Journal of Heat and Mass transfer, vol. 31, no.3: p664-667
[20] Suslick, K.S., 1998, Ultrasound: Its Chemical, Physical and Biological Effects. VCH Publishers, New York, U.S.A
[21] Wong, S.W., Chon, W.Y., 1969, Effects of Ultrasonic Vibration on Heat Transfer to Liquids by Natural Convection and by Boiling. AICHE Journal, vol. 15, no.2: p281-288
[22] Yao Ye; Zhang, Xingyu; and Guo, Yiying, 2010, Experimental Study on Heat Transfer Enhancement of Water-water Shell-and-Tube Heat Exchange Assisted by Power Ultrasonic" (2010). International Refrigeration and Air Conditioning Conference. Paper 1110. http://docs.lib.purdue.edu/iracc/1110
[23] B.G.W. Yee, J.C. Couchman, 1976, Application of ultrasound to NDE of

materials, IEEE Transactions on Sonics and Ultrasonics SU-23, vol. 5, pp. 299-305.
In most of the prior art, a great deal of success has been obtained by Ultrasonic measurements employed in non-destructive evaluation and the detailed work are delineated in the overview of measurement techniques by Yee [24]. Another application of ultrasound is ultrasonic pyrometry as used in many process control systems, detailed in the work of Hoyle and Luke [6], because the sound speed is a function of temperature in most materials.
The pioneering work of acoustic streaming forced convection was performed by Fand and Kave [5] who expected heat transfer enhancement. In a detailed survey on the techniques to enhance heat transfer with ultrasonic vibrations, Bergles [2] reported the significant increases in nonboiling heat transfer at moderate flow velocity due to the cavitation by the previous researchers. Difficulties to locate the transducer so as to obtain good coupling with the fluid and the drawback of minimum attenuation of the ultrasonic energy by the vapour is also reported in the work of Bergles and Newell [3]. In his Ph. D. dissertation Larson [9] established that the cavitation was responsible for the increase in Nusselt number at the low frequencies in the natural convection flows over a sphere while the effect of variations of ultrasonic frequencies, Nusselt number and Reynolds numbers were studied. Though acoustic streaming was found to be the major factor of enhancement at higher frequencies, no sufficient increase in heat transfer was obtained to justify the use of ultrasound as a means of heat transfer. An increase of a 300 percent in the heat-transfer coefficient in natural convection was obtained in for a study of heat transfer from

a sphere to water and toluene where the liquid was subjected to ultrasonic agitation. During study of the forced convection with the same set up and Reynolds number of 400, Larson and London [10] achieved an increase in the Nusselt number up to 60% for a frequency up to 125 kHz.
Several studies were made in the areas of natural convection by Fand, [4]; Li and Parker [14]; Wong and Chon [21]; lida and Tsutsui, [15]; and also in the area of pool boiling heat transfer by Wong and Chon [21]; lida and Tsutsui, 1992 [15]; Park and Bergles [19]; Bonekamp and Bier, [1]. The acoustic streaming induced by ultrasound flexural vibrations (that increased the turbulence intensity of air near the plate surface) has been attributed to be the cause of convective heat transfer enhancement under gaseous environment by Hyun et al. (2005)[7] during the study of the convective heat transfer by ultrasound under gaseous environment and it resulted in increase of the turbulence intensity of air near the plate surface. Kim et al.[8] employed the flow visualization and thermal measurement to study the relationship between the flow behavior induced by ultrasonic vibration and the consequent heat transfer enhancement in natural convection and pool boiling regimes. The study of the flow behavior induced by ultrasonic vibration and the consequent heat transfer enhancement in natural convection and pool boiling regimes by Kim et al [8] exhibited the degree of heat transfer enhancement in the natural convection and sub-cooled boiling regime by the behavior of cavitation bubbles. It was again established that acoustic streaming was the major factor enhancing the heat transfer rate. Up to a tenfold increase in heat transfer coefficient was achieved due to acoustic streaming in experimental work of Nomura et al. [18] during a study of the influence of streaming induced by ultrasound vibration on heat transfer in liquid environment

(water) using a horn-type ultrasonic vibrator with 60.7 kHz. No significant favorable experimental results were achieved in the study of Yao et al. [22] during test on the heat transfer enhancement of water-water heat exchanger in shell-and-tube type assisted by power ultrasonic. The acoustic frequency of ultrasound transducer employed was about 21 kHz while three power levels (40W, 60W and 100W) were used for this study. It was found that the water flow rate and ultrasound power levels would produce great influence on the enhancement by power ultrasound which decreased with the increasing water velocity in the tube and the decreasing acoustic power.
Thus, the above-mentioned prior art suggest that the there is a considerable research interest to the heat transfer due to the imposition of ultrasonic wave in fluid flow or flow boiling. Application of ultrasonic in the area of detection of crack and other non destructive experiments are already well known as described in the literature. Accordingly, the prior art findings have stimulated a concept that the ultrasonic waves are indeed a stimulator for better heat transfer mechanism. However, there is hardly any significant result or findings from any experimental work with mixed convection coupled with ultrasonic wave for enhancement of heat transfer can be found in laid open publications.
OBJECTIONS OF THE INVENTION
It is therefore an object of propose an improved process for enhancement of heat transfer through mixed convective heat transfer in a reservoir augmented by application of ultrasonic waves.

Another object of the invention is to propose a system for mixed convective heat transfer in a reservoir by ultrasonic wave application through the fluid medium to achieve a faster heating rate inside the reservoir.
A further object of the invention is to propose a system for mixed convective heat transfer in a reservoir by ultrasonic wave application through the fluid medium, in which the fluid medium is disallowed to come into direct contact with the ultrasonic generator.
SUMMARY OF THE INVENTION
It is known that the heat conductivity of water is generally not very high. According to the invention, water as a common fluid is used for the working medium and is heated inside a chamber followed by stirring of the fluid. A transducer attached to the body of the heat reservoir is used to impart ultrasonic wave to the fluid medium. The mixed convection type of heat transfer inside the reservoir is boosted by the bubbles generated through the ultrasonic wave that is produced inside the hot water. The ultrasonic wave through the mixed convection raises water temperature. This is an enhancement which has a potential for high temperature increase.
According to the invention, the system comprises a hot reservoir unit, containing at least one chamber for heating, and a transducer for imparting ultrasonic wave cooling, the water acting as a convective heat transfer medium.

The step of the inventive process are as under:
(1) filling the system with water;
(2) keeping the drain valve at the bottom of the chamber closed;
(3) activating a heater to heat the water up to 70 degree C and stirring the water inside the chamber;
(4) recording temperature of the water and leaving the heat exchanger, and observing the water flow rate in the hot water loop for every minute;
(5) turning the ultrasonic generator on, and adjusting the power level of the ultrasound that is produced by the ultrasound transducer;
(6) continuing recording of the variables included in step (5); and
(7) repeating steps (3)-(6) for the other conditions.
The time of the ultrasound exposure is an important factor in these processes. For a short ultrasound exposure, the initial nuclei population and size distribution is of crucial importance because these parameters are not likely to change much during the exposure. Thus, for process aimed at determining the kinetics of cavitation-added physical or chemical processes, the source of cavitation nuclei, which determines the initial population and size distribution of the cavitation nuclei, is a parameter of paramount importance. The process has to be implemented with ultrasonic wave of short exposure as well as of large exposure time.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1 - Shows a heat reservoir system according to the invention

Figure 2 - Shows one dimensional view of a reservoir through which an ultrasonic pulse propagates including an ultrasonic wave generator.
DETAILED DESCRIPTION OF THE INVENTION
Preparation of the system:
The present invention will be better understood from the following brief
description of the system reference to the accompanying drawings.
As shown in Figure 1, the heat reservoir system is provided with a single chamber (1) having a heating unit (3) for heating the fluid or water.
In the reservoir ®, at least one heater (3) is provided for heating water/fluid desired to be used. The heater (3) is placed at a distance away from the adjoining surfaces of the chamber (1) such that the heater does not touch the surface of the chambers (1).
Two RTDs (4,4) one at the top of the reservoir (R) and One RTD (4) at the bottom of the reservoir (R), close to the separating plate are provided. The chamber (1) can be emptied as necessary.
The base of the heat reservoir (R) is located at a higher level such that drainage of liquid from the chamber (1) is facilitated. A separate mounting/framework holds motor-stirrer assembly (5,6). Proper insulation around the outer surface of the chamber (1) ensures minimization of the heat loss. Plug points and switches

along with fuse in a switch board (7) are provided for RTDs (4,4), the heater (3) and the stirrer (6).
Following the filling of the chamber (1) by water, the water is heated and this is followed by stirring by the motor -stirrer assembly (5,6). The stirring ensures the achievement of the uniform temperature inside the chamber (1). An ultrasonic generator (8) is operably connected to a transducer (7) disposed in the reservoir. Accordingly, ultrasonic waves are applied to the liquid/fluid. The power is increased for the ultrasound wave that is transmitted through the surface of the wall. The transducer (7) transmits the wave to the water medium kept inside the chamber (1). The recording of the temperature the RTDs (4,4) at a regular interval ensures the monitoring the trend of the temperature history over a period of time due to the ultrasonic wave. Similar to the above, the power of the ultrasonic wave is varied to notice the effect of the intensity of the ultrasound on this system.
The chamber/reservoir has a top cover and a bottom cover each having at least one RTD. Each RTD is provided with a digital display means for displaying temperature data, and the digital display means are connected to a datalogger. A controller with a cutoff relay is provided to switch off the heater when the display means relating to the RTD exhibits a temperature exceeding a threshold value. The electric motor has a regulator. A hole passage is provided on the top cover (TC) of the chamber. The transducer is fitted to one wall (outside) of the chamber externally. The transducer is operably attached to the ultrasound wave generator. The intensity of the ultrasound waves can be varied.

Before application of the ultrasonic wave, the temperature increased only through the heater (3) in the reservoir (R) is T2 from cold water temperature (T1) If a resultant temperature of T3 is achieved with augmentation of ultrasonic wave and the mixed convection in the reservoir from the cold water temperature T1, then effectiveness (s) of the ultrasonic wave and mixed convection, is represented by the relationship of:


WE CLAIM
1. A system for mixed convective heat transfer in a reservoir augmented by
application of ultrasonic wave,
- a heat reservoir having at least one chamber, the chamber accommodating water for heating by a heater;
- a heating means for heating the water;
- a transducer disposed external to the reservoir, and operably connected to an ultrasound generator;
- at least one each RTD is disposed on the top and bottom of the reservoir to measure water temperature at several time intervals; and
- a controller is provided to regulate the heating means corresponding to temperature exhibited by the RTDs and a threshold value of temperature pre-stored.

2. The system as claimed in claim 1, wherein the reservoir has top and bottom covers which each his provided with at least one RTD.
3. The system as claimed in claim 2, wherein each RTD comprises a digital display means, and wherein the display means are connected to a datalogger.
4. The system as claimed in claim 1, wherein a hole passage is provided on the top cover of the chamber.
5. The system as claimed in claim 1, wherein the hot reservoir comprises of a stirrer provided with a motor including a shaft rod for stirring and mixing

the contents in the reservoir, and wherein the motor is equipped with a regulator.
6. The system as claimed in any of the preceding claims, wherein the intensity of the ultrasound waves is variable.
7. A system for mixed convective heat transfer in a hot reservoir augmented by application of ultrasonic waves as substantially described and illustrated herein with reference to the accompanying drawings.

ABSTRACT

The invention relates to a system for mixed convective heat transfer in a reservoir augmented by application of ultrasonic wave, a heat reservoir having at least one chamber, the chamber accommodating water for heating by a heater; a heating means for heating the water; a transducer disposed external to the reservoir, and operably connected to an ultrasound generator; at least one each RTD is disposed on the top and bottom of the reservoir to measure water temperature at several time intervals; and a controller is provided to regulate the heating means corresponding to temperature exhibited by the RTDs and a threshold value of temperature pre-stored.