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Improvement of heat transfer by means of ultrasound: Application to a double-tube heat exchanger
Ultrasonics Sonochemistry 19 (2012) 1194–1200
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Ultrasonics Sonochemistry journal homepage: www.elsevier.com/locate/ultson
Improvement of heat transfer by means of ultrasound: Application to a double-tube heat exchanger M. Legay a, B. Simony a, P. Boldo b, N. Gondrexon a,c,⇑, S. Le Person d, A. Bontemps d a
LEPMI, UMR 5279, CNRS – Grenoble INP – Université de Savoie, Université Joseph Fourier BP 75, 38402 Saint Martin d’Hères, France EDYTEM, UMR 5204, Campus scientifique – Université de Savoie, 73376 Le Bourget du Lac Cedex, France c Laboratoire Rhéologie et Procédés, UMR 5520, CNRS – Université Joseph Fourier Grenoble I, Grenoble-INP BP 53, 38041 Grenoble Cedex 9, France d LEGI, UMR 5519, Domaine Universitaire BP 53, 38041 Grenoble Cedex 9, France b
a r t i c l e
i n f o
Article history: Received 18 October 2011 Received in revised form 3 April 2012 Accepted 3 April 2012 Available online 11 April 2012 Keywords: Ultrasound Heat transfer Heat exchanger Enhancement
a b s t r a c t A new kind of ultrasonically-assisted heat exchanger has been designed, built and studied. It can be seen as a vibrating heat exchanger. A comprehensive description of the overall experimental set-up is provided, i.e. of the test rig and the acquisition system. Data acquisition and processing are explained step-by-step with a detailed example of graph obtained and how, from these experimental data, energy balance is calculated on the heat exchanger. It is demonstrated that ultrasound can be used efficiently as a heat transfer enhancement technique, even in such complex systems as heat exchangers. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction, literature review It is necessary to go back to the 60s to find the first reported studies where ultrasound was used as an active heat transfer enhancement method [1]. At first, it used to concern only convection heat transfer; but results obtained did not seem promising enough to continue investigations on this subject [2,3]. So, the use of ultrasound was then extended to phase change phenomena, successfully gaining in speed during melting for example [4,5]. Boiling heat transfer is another important research field [6–9], where ultrasonic vibrations promote bubbles formation and therefore delay the onset of the critical heat flux. More recently, investigations began to head toward fouling reduction for heat exchangers [10,11], whereas convection is becoming again a subject of interest [12–14]. An undeniable advantage of ultrasound is its cumulative effects leading to several improvements on a single system. Heat exchangers are perfectly suited examples to illustrate this doubly favorable factor, with both heat transfer enhancement and cleaning possibilities [15].
⇑ Corresponding author at: Laboratoire Rhéologie et Procédés, UMR 5520, CNRS – Université Joseph Fourier Grenoble I, Grenoble-INP BP 53, 38041 Grenoble Cedex 9, France. Tel.: +33 4 76 82 65 90; fax: +33 4 76 82 67 77. E-mail addresses: [email protected] (M. Legay), [email protected] (B. Simony), [email protected] (P. Boldo), [email protected] (N. Gondrexon), [email protected] (S. Le Person), [email protected] (A. Bontemps). 1350-4177/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.ultsonch.2012.04.001
However, only a few authors have studied vibrating heat exchangers although very promising results were reported [16,17]. These reasons already led us to develop and study three different heat exchanger configurations to observe the influence of ultrasound in such systems. All of them include two fluids, contrary to most studies in the literature where only one fluid is involved. The first system of interest was a sonochemical reactor containing hot water at rest into which was inserted a copper coil [18]. In this coil was flowing cold water to chill out the reactor content, with and without ultrasound and at frequencies varying from 20 kHz to 1.6 MHz. It was found that the heat transfer coefficient could be increased up to two times, resulting in the enhancement of the cooling rate of the hot water volume. The second system designed was a shell-and-tube heat exchanger where both hot and cold fluids were flowing [19]. Using a special device accurately described thereafter (the SonitubeÒ); it was possible to make the heat exchanger structure resonate at 35 kHz. The overall heat transfer coefficient of this shell-and-tube heat exchanger was enhanced up to 260%. Finally, the third configuration of interest is the one presented in this work: a double-tube heat exchanger. A thermal approach on a slightly different geometry has already been presented and improvements are of the same order as for the shelland-tube heat exchanger [20]. Consequently, the new objectives here are to present a full description of the methods and equipment used and to show as simply as possible that ultrasound, due to its well-known induced effects, is also a valuable technique for heat transfer enhancement inside heat exchangers.
M. Legay et al. / Ultrasonics Sonochemistry 19 (2012) 1194–1200
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Nomenclature Latin and Greek symbols A area (m2) Cp specific heat (J kg1 K1) D diameter (m) h convection heat transfer coefficient (W m2 K1) k thermal conductivity (W m1 K1) L length (m) _ m mass flow rate (kg s1) P power (W) q, Q heat flow rate (W) R thermal resistance (K W1) t time (s) T temperature (K) U overall heat transfer coefficient (W m2 K1) U diameter of the SonitubeÒ (m)
2. Description of the system 2.1. Vibrating heat exchanger The vibrating heat exchanger of interest is a double-pipe configuration, i.e. it is made of two concentric straight pipes inserted one into the other. The largest one, external, is a device well-known to sonochemists: the SonitubeÒ (model SM35) [21]. It can be put into vibration at 35 kHz by a piezoelectric transmitter fixed at its center. Besides, its shape was designed with the aim to make it resonate and communicate powerful ultrasonic vibrations to the fluid flowing inside [22]. Indeed, this device was first developed for industrial applications in order to treat a continuous flow of solution by power ultrasound. In this work, it was adapted to build a double pipe heat exchanger by including a straight pipe into it, as schematically shown in Fig. 1. The version of the so defined sono-exchanger used in this work is a modified one compared to that reported for the first time in [20]. The diameter of the internal pipe was reduced in order to ensure a better stability of the generator that supplies ultrasonic power to the system. Two non-vibrating elements were added at each extremity of the vibrating tube to distribute and to collect the fluid at the inlet and outlet of the annular gap. Hot water always flows into the central pipe whereas cold water always flows throughout the annular space. However, the hydrodynamic configuration can be either parallel-flow or counter-flow. The central pipe has a 7 mm internal diameter and is 0.5 mm thick whereas the vibrating tube is 20 mm in internal diameter. The vibrating length is 250 mm and the total length, where heat exchange takes place, is 340 mm. 2.2. Test rig In order to operate the heat exchanger, it was necessary to build all the experimental setup surrounding it (piping, heater, acquisi-
DT
temperature difference (K)
Subscripts amb ambient c cold side, cold water env environment exc exchanged ext external side h hot side, hot water in at the inlet int internal side lm log-mean out at the outlet US ultrasound, in the presence of ultrasound
tion system, etc.). Fig. 2 sketches a simplified version of the global configuration used to run and analyze performances of this vibrating heat exchanger. Compared to the one presented in [20], this experimental setup involves modifications. A volumetric pump and a buffer tank were added on the cold fluid circuit in order to avoid water flow rate fluctuations as it already existed on the hot fluid circuit. This setup is composed of two water circuits: an opened one for cold water and a closed one for hot water. Cold water directly comes from the domestic water supply at a temperature of 15 °C; part of it is stored into the buffer container to ensure a sufficient supply to the pump inlet. Then, cold water flows into the heat exchanger, is heated up and eventually rejected to the sewer. The circuit is almost the same for the hot water except that, once the tank has been filled, water is not rejected and flows in a loop circuit. Therefore, it is continuously re-heated up to 37 °C in the tank to ensure heat exchange with cold water in the heat exchanger. 2.3. Acquisition system Water flow rates are measured by McMillan turbine flowmeters and temperatures are measured by Pt 100 probes (RTD sensors – Resistance Temperature Detector). These apparatus are positioned as indicated in Fig. 2, i.e. the flowmeters are set after the pumps and the temperature sensors are at the inlets and outlets of the heat exchanger. Two other temperature sensors (only useful as indicators but not for calculations) are set in the hot water tank and in the surrounding air. These sensors are connected to a data acquisition module ADAM 4015 whereas the flowmeters are connected to a data acquisition module ADAM 4018. A third and last module is an ADAM 4561 and it serves as a link to the computer to convert RS485 signals to USB. A VBA macro acquires and processes the data in real time.
Fig. 1. Sketch and dimensions of the vibrating double pipe heat exchanger: dimensions in millimetres.
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Fig. 2. Configuration of the global system: simplified schematic diagram.
pipe is set to 0 L/min and only cold water is flowing. If a basic energy balance is performed onto the system working in steady state, it will be found that the ultrasonic power delivered P US is thermally recovered by the cold fluid flowing in the annulus as written in Eq. (1):
_ c CpDT c PUS ¼ m
Fig. 3. Acquisition system setup.
Results are displayed and stored in a spreadsheet file. This acquisition system is sketched in Fig. 3. The VBA macro is set to collect data: the hot and cold fluid flow rates and the four temperatures at the inlets and outlets of the heat exchanger, at regular interval (usually each 2 or 3 s). These data are stored and processed in a spreadsheet document file. Hot and cold heat flow rates, temperature differences and overall heat transfer coefficients are therefore computed and displayed in real time. The uncertainty measurements were estimated at a maximum of 3.1% for the flow rates (according to the measurements and to the flowmeters supplier’s handbook) and at 0.2% for the temperatures.
ð1Þ
PUS is the ultrasonic power dissipated into heat in the cold fluid, computed by multiplying its mass flow rate by the specific heat of the water Cp and by the temperature difference DT c between the inlet and the outlet of the heat exchanger. Fig. 4 shows the evolution of the ultrasonic power transmitted as a function of cold water flow rate in the annulus. Average values obtained from 3 different measurements are plotted on this graph. Experiments were even repeated with the central pipe completely empty of water (filled with air to change the acoustic impedance) and results were the same. This estimation of the ultrasonic power dissipated into heat in the annulus is necessary to fulfill the energy balances when both fluids are flowing. Except for very low water flow rates in the annulus (e.g. 0.2 L/ min, PUS 50 W), the ultrasonic power transmitted is quite stable:
3. Methodology for results analysis 3.1. Estimation of the ultrasonic power transmitted Before beginning the heat transfer experimentations, it is necessary to estimate the ultrasonic power delivered to the fluid. It is assumed that all the mechanical energy of the waves is ultimately dissipated into heat at the annulus side (in the cold fluid in Fig. 1). It is therefore possible to evaluate the amount of ultrasonic power transmitted to this fluid, by calorimetric measurement in a dynamic mode. To do so, the hot water flow rate inside the central
Fig. 4. Calorimetric measurements: average ultrasonic power transmitted to the cold fluid in the annulus versus its flow rate.
M. Legay et al. / Ultrasonics Sonochemistry 19 (2012) 1194–1200
around 70–75 W. It means that the water flow rate has very little influence on the energy dissipated in the heat exchanger for this geometry. The lower ultrasonic power transmitted, at low water flow rates, is explained in part by automatic adjustment of the power delivered by the Sonitube’s generator. There is indeed a minimal fluid velocity for good frequency resonance matching between the transducer, the vibrating structure and the fluid flow as explained in the Sonitube supplier’s handbook. The generator output power was never modified manually during the tests and was kept constant at its minimum value, i.e. 100 W at the most. In fact, the generator can deliver up to 400 W but it is manually set to 50% amplitude and besides, a de-booster is used after the piezoelectric transducer, reducing again the amplitude by half before entering the structure (sonotrode). It was necessary to undertake such settings in order to have the best heat exchanger energetic efficiency as possible. To reduce more the ultrasonic power was unfortunately impossible since otherwise, the generator power output would not be sufficient to start the vibration of the whole structure. 3.2. Typical interpretation of collected data In addition to the computing and recording of collected values, the software is also capable of plotting in real time the evolution of the four temperatures (2 inlets and 2 outlets) versus time. It is therefore possible to see immediately on a graph when the heat transfer process reaches its steady state (usually only a few minutes). An example of typical graph obtained is proposed in Fig. 5. At initial time t = 0 s when the acquisition program is launched, the steady state of the heat transfer process under silent conditions has already been reached. That is why the four temperatures are stable until t = t1, i.e. they are not evolving in time if no change (e.g. flow rate or energy supply) is applied to the system. The temperature difference is greater for the cold water flow since its flow rate is lower than the hot water one and the heat flux exchanged is equal for both fluids (see Section 3.3 for equations and explanation). At time t1 40 s, the ultrasound generator is turned on and the heat exchanger structure is put into vibration. The two inlet temperatures of hot and cold water (‘‘cold in’’ and ‘‘hot in’’ in Fig. 5)
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are not affected by ultrasound since the sensors are set just before the inlets of the heat exchanger, so these flows of water continue to arrive at the same temperature. Conversely, the two outlet temperatures (‘‘cold out’’ and ‘‘hot out’’ in Fig. 5) are changed to a great extent: the cold one becomes hotter whereas in the meantime, the hot one is slightly lowered. This clearly shows an improvement of the heat exchange rate. The increase of the cold fluid outlet temperature is notably more important than the decrease of the hot fluid outlet temperature. This is explained by the ultrasonic power dissipated into heat at the cold side, increasing the temperature difference. It is also due to the lower mass flow rate (ratio 0.5/ 1.6): to collect the same amount of energy, a larger temperature difference is needed (see for instance Eq. (1)). At time t2 135 s, ultrasound is turned off and the temperatures at the outlets regain their initial values without ultrasound. 3.3. Energy balance In order to analyze the heat exchanger performances, energy balances with and without ultrasound are performed. Fig. 6 shows a schematic representation of the system considered, in a parallelflow configuration. Without ultrasound, the energy balance is obvious (Eq. (2)): it says that the power transferred by the hot water flow corresponds to the power received by the cold water flow (assuming no heat transfer to the environment). It is also called the exchanged heat flow rate qexc .
qc ¼ qh ¼ qexc
ð2Þ
Eq. (2) describes what happens between t0 and t1 in the graph Fig. 5. Another term, corresponding to the heat transfer rate to the environment qenv , is sometimes added. Therefore, the heat exchanger cannot be considered as perfectly adiabatic, although it is very well insulated. qenv is always negligible, i.e. inferior to 10% of qexc . Taking qenv into account, Eq. (2) then becomes Eq. (3).
qc þ qenv ¼ qh
ð3Þ
qenv is generally negative (with respect to its corresponding arrow in Fig. 6, i.e. it will contribute to the heating of the cold water flow) since T amb > T c , which is usually the case in this work. Indeed, T amb was between 20 and 25 °C during all the series of experiments whereas the cold water temperature is around 15 °C in average. With ultrasound, the energy balance is written as in Eq. (4):
qc þ qenv ¼ qh þ PUS
ð4Þ
The ultrasonic power transmitted PUS is a positive quantity (because entering the system) estimated previously for each flow rate as explained in Section 3.1. qc and qh can be calculated respectively with Eq. (5) and Eq. (6), with or without ultrasound:
_ c CpðT c;out T c;in Þ qc ¼ m _ h CpðT h;in T h;out Þ qh ¼ m
ð5Þ ð6Þ
In the presence of ultrasound (or also when there is heat flow to or from the environment), qc and qh are not equal any more. Therefore, it is qh that is referred to as qexc in order to avoid taking into account the term corresponding to ultrasonic energy dissipation.
Fig. 5. Evolution of the four temperatures versus running time, without and with ultrasound. Cold water flow rate in the annulus: 0.5 L/min, hot water flow rate in the tube: 1.6 L/min.
Fig. 6. Energy balance on the system considered.
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4. Results and discussion 4.1. Chart bar presentation of the energy balance From the energy balance of Eq. (4), it is possible to draw chart bar diagrams to distinguish better the enhancement brought by ultrasonic vibrations. The amounts reported in the bars are the instantaneous energy transfer rates obtained when the heat exchanger is working in steady state regime. Fig. 7 shows an example of energy balance obtained without and with ultrasound, for a cold water flow rate equal to 0.5 L/min and a hot water flow rate equal to 1.6 L/min, in a parallel flow configuration. In this example, there is no heat flow rate to the environment without ultrasound. The heat flow rate exchanged is equal to 128 W: the two bars are precisely at the same level, which exactly corresponds to Eq. (2). With ultrasound, the four quantities of the energy balance have been plotted side by side. It is then possible to virtually superpose the bars in order to correspond exactly to the energy balance of Eq. (4) and to obtain once again two bars up to the same level (each side of Eq. (4)). Thus, the energy balance is fulfilled: 273 W at each side, meaning some enhancement owing to the effects of vibrations. By subtraction, one can easily quantify the improvements: bringing 67 W of ultrasonic power (computed by calorimetric measurement as explained in Section 3.1), the exchanged heat flow rate (qexc ¼ qh ) is increased by 78 W (206– 128) and qc is increased by approximately the same amount plus the ultrasonic power dissipated (also ±qenv in reality, i.e. 130 W total increase in this example). In the presence of ultrasound, the heat flow rate to the environment qenv is positive, which means that there are heat losses (same direction as the arrow in Fig. 6). It can also be due to an over-estimation of the ultrasonic power transmitted that can sometimes fluctuate around the value estimated in the calorimetric section. Nevertheless, qenv is still considered negligible for being inferior to 10% of the heat flow rate exchanged. Fig. 8 shows the same kind of diagram for the same water flow rates but in a counter-current configuration. The only parameter that has been changed compared to the previous example in Fig. 7 is the direction of hot water circulation. Without ultrasound, heat flow rates are higher than in parallelflow configuration, which is a typical and well-known observation due to the more elevated temperature difference between the fluids all along the heat exchanger. The heat flow rates at the hot and
Fig. 7. Example of energy balance in a chart bar form: cold water flow rate 0.5 L/ min, hot water flow rate 1.6 L/min, parallel flow configuration.
Fig. 8. Chart bar presentation of the energy balance: cold water flow rate 0.5 L/min, hot water flow rate 1.6 L/min, counter-flow configuration.
cold sides are almost equal although there is a negligible heat flow rate from the environment (see Eq.(3)): 5 W, contributing to heating the cold fluid (direction opposite to the arrow of Fig. 6). With ultrasound, improvements in qh and qc , 63 and 136 W, are also relevant in this counter-current configuration since the ultrasonic power delivered is equal to 67 W. The heat flow rate to the environment is still negative (heating the cold water flow) and negligible compared to qexc . From Figs. 7 and 8, it is possible to conclude that heat transfer enhancement is not only due to the ultrasonic power dissipated into heat at the cold side. Ultrasound has side-effects enhancing heat transfer, discussed in Section 4.2. 4.2. Effects of ultrasound on the overall heat transfer coefficient The attention is now turned to the analysis of the overall heat transfer coefficient U, to try to find explanations to the improvements. Experimental values of U are calculated with Eq. (7):
U¼
qexc ADT lm
ð7Þ
Fig. 9. Overall heat transfer coefficient versus cold water flow rate in the annulus: influence of flow configuration (hot water flow rate fixed: 1.6 L/min).
M. Legay et al. / Ultrasonics Sonochemistry 19 (2012) 1194–1200
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where DT lm is the log-mean temperature difference and A is the area across which heat transfer occurs. Remembering also that qexc is taken equal to qh to avoid an overestimation of the improvements (the hot fluid is in the central pipe, not directly subjected to ultrasound or to heat transfer to/from the environment). This coefficient gives a global view of heat exchanger performances, which is why it is of great interest. Fig. 9 presents the evolution of U for four different configurations: parallel and counter-flow, both with and without ultrasound. Four important observations have to be made about Fig. 9:
the annulus for the following interpretation. In the form of a theoretical equation, the overall heat transfer coefficient U of a doubletube heat exchanger can be written as the sum of three thermal resistances, as in Eq. (8):
– There is no notable difference between the overall heat transfer coefficients in parallel-flow or counter-flow configurations for the same flow rates considered. This is easily explained: no change in flow rates imply no change in convection coefficients and therefore no modification of the overall heat transfer coefficient (see also Eq. (8) thereafter). The highest heat exchange flow rates usually obtained in counter-flow configurations are only due to the higher temperature difference across the heat exchanger. – The improvements in the presence of ultrasound are noticeably the same for both configurations. Indeed, there is no apparent reason why intensification would be greater if water flows in one direction or another, if the flow rates and the other parameters do not change. – Ultrasonic intensification is more important for low cold water flow rates. A possible interpretation of this observation is given by Fig. 10. – Conversely, in a transition/turbulent flow (P3 L/min), the ultrasonic improvements are reduced. Explanation following Fig. 10 gives a potential reason.
The three terms on the right side of the equation correspond to each thermal resistance respectively named R1, R2 and R3. D is the diameter of the tube, L is its length, k is the thermal conductivity of the metal and h is the convective heat transfer coefficient. Fig. 10 shows a zoom of the internal pipe thickness, in the longitudinal cross-section, to visualize each of these thermal resistances. For this schematic drawing, a laminar flow has been chosen in the annulus side and a turbulent flow in the pipe side. This corresponds to the configuration where the highest improvements were observed [19], and to the previous examples (Figs. 5 and 7–9). There are three zones represented in Fig. 10, corresponding to the three thermal resistances:
In the heat exchanger studied, ultrasound is applied to the shell (external tube). Therefore, the effects are likely to be much more intense in the shell side (annulus) than inside the internal tube. This is supported in Section 3.1 by the fact that no change were observed when measuring the ultrasonic power transmitted with the central pipe either empty or filled with water at rest. If water in the central pipe was affected by ultrasound (e.g. absorbing part of the ultrasonic power delivered), PUS would probably be modified. This is the reason why effects are assumed negligible elsewhere than in
1 ¼ R1 þ R2 þ R3 UA
ð8Þ
Expressing each resistance term with geometrical and physical parameters, Eq. (8) becomes Eq. (9):
1 1 lnðDext =Dint Þ 1 þ ¼ þ UA h1 A1 2pkL h 3 A3
ð9Þ
– The annulus side, where the cold water flow is laminar and the thermal resistance (R1) is therefore very important (without ultrasound) due to the low velocity of water. – The pipe thickness (0.5 mm, stainless steel), where conduction heat transfer takes place. The thermal resistance (R2) is always negligible compared to the other ones. – The internal pipe side, where the liquid flow is turbulent and the thermal resistance (R3) is consequently assumed lower than the one in the annulus (but still higher than the thermal conduction resistance). It can be seen on Fig. 10 that ultrasound, transmitted in the annulus, has the effect to decrease R1 to R1US by enhancing the convection heat transfer coefficient h1. Without ultrasound, R1 is the limiting thermal resistance, i.e. the one that hampers heat transfer the most. By decreasing it, the overall heat transfer coefficient is increased and heat exchange process is enhanced globally.
Fig. 10. Illustration of ultrasound effects: heat transfer enhancement inside the heat exchanger.
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The thermal conduction resistance in the pipe thickness R2 and the convection resistance in the internal pipe R3 are assumed unchanged by the ultrasound. The common ultrasonically-induced phenomena are usually held responsible for these improvements. Acoustic cavitation, and especially bubbles oscillation and powerful implosion, creates micro-turbulence at the solid–liquid interface and breaks the dynamic boundary layer. The fluid is therefore mixed and the convection phenomenon is strongly intensified. Vibrations of the walls and inside the fluid are also likely to have some non-negligible influence on the fluid stirring. This would also explain why when the flow is already turbulent in the annular space, improvements are less pronounced (right side of Fig. 9). In this work, acoustic streaming, usually reported to have great influence on heat transfer [23,24], is likely to be negligible since fluids are already flowing due to the pumps. 5. Conclusion An ultrasonically-vibrating tube-in-tube heat exchanger has been studied in this work. This new type of heat exchanger was inserted within a test rig that has been thoroughly detailed in this paper. In particular, the data acquisition system specifically developed for this work has been reported. It enables to follow the evolution versus running time of the temperatures at the inlet and the outlet of the heat exchanger for both fluids. The simple observation of the graph obtained is a qualitative way to illustrate the improvement of heat transfer in the presence of ultrasound. In a second step, an energy balance is performed based upon data collected in steady-state operation of the heat exchanger. This energy balance is expressed through classical thermal equations, taking into account the ultrasonic power provided to the fluid in the annular space, estimated by calorimetric measurements. Bar graphs were used to illustrate values of this energy balance and to demonstrate the enhancement in the presence of ultrasound. Finally, overall heat transfer coefficients were classically determined with and without ultrasound. It has been shown that, whatever the heat exchanger flow configurations, the overall heat transfer coefficients in the presence of ultrasound are higher than under silent conditions. It was assumed that the conduction thermal resistance through the thickness of the internal pipe and that the convection thermal resistance inside the internal pipe are not affected by ultrasonic waves. It was therefore concluded that only the convection thermal resistance in the annular space was decreased, leading to an enhanced exchanged heat flow rate. It is considered that physical effects induced by cavitation bubbles implosion as well as vibrations of the walls result in a disturbance of the dynamic boundary layer. To conclude, ultrasound is shown here to be an interesting way to improve heat exchanger performances. Further studies would consist in investigating the potential of ultrasound for fouling reduction in heat exchangers.
Acknowledgement This work was supported by the Association Institutes Carnot, Energies du futur. References [1] M. Legay, N. Gondrexon, S. Le Person, P. Boldo, A. Bontemps, Enhancement of heat transfer by ultrasound: review and recent advances, Int. J. Chem. Eng. (2011) 1–17, doi:http://dx.doi.org/10.1155/2011/670108. [2] A.E. Bergles, P.H. Newell Jr., The influence of ultrasonic vibrations on heat transfer to water flowing in annuli, Int. J. Heat Mass Trans. 8 (1965) 1273– 1280. [3] M.B. Larson, A study of the effects of ultrasonic vibrations on convective heat transfer in liquids, Ph.D. Dissertation, Stanford University, Mic, 1961, pp. 61– 1235. [4] H.V. Fairbanks, Influence of ultrasound upon heat transfer systems, Ultrason. Symp. (1979) 384–389, http://dx.doi.org/10.1109/ULTSYM.1979.197226. [5] Y.K. Oh, S.H. Park, Y.I. Cho, A study of the effect of ultrasonic vibrations on phase-change heat transfer, Int. J. Heat Mass Trans. 45 (2002) 4631–4641. [6] S.W. Wong, W.Y. Chon, Effects of ultrasonic vibrations on heat transfer to liquids by natural convection and by boiling, AIChE J. 15–2 (1969) 281–288. [7] K.A. Park, A.E. Bergles, Ultrasonic enhancement of saturated and subcooled pool boiling, Int. J. Heat Mass Trans. 31–33 (1988) 664–667. [8] S. Bonekamp, K. Bier, Influence of ultrasound on pool boiling heat transfer to mixtures of the refrigerants R23 and R134A, Int. J. Refrig. 20–8 (1997) 606– 615. [9] F. Baffigi, C. Bartoli, Heat transfer enhancement from a circular cylinder to distilled water by ultrasonic waves in subcooled boiling conditions, Proc. ITP, October 4–9, Volterra, Italy, 2009. [10] T.R. Bott, Biofouling control with ultrasound, Heat Trans. Eng. 21 (2000) 43–49. [11] T.R. Bott, L. Tianqing, Ultrasound enhancement of biocide efficiency, Ultrason. Sonochem. 11 (2004) 323–326. [12] M.K. Aktas, B. Farouk, Y. Lin, Heat transfer enhancement by acoustic streaming in an enclosure, J. Heat Trans. 127 (2005) 1313–1321. [13] S. Hyun, D.R. Lee, B.G. Loh, Investigation of convective heat transfer augmentation using acoustic streaming generated by ultrasonic vibrations, Int. J. Heat Mass Trans. 48 (2005) 703–718. [14] S. Nomura, M. Nakagawa, S. Mukasa, H. Toyota, K. Murakami, R. Kobayashi, Ultrasonic heat transfer enhancement with obstacle in front of heating surface, Jpn. J. Appl. Phys. 44(6B) (2005) 4674–4677. [15] W. Benzinger, U. Schygulla, M. Jäger, K. Schubert, Anti-fouling investigations with ultrasound in a microstructured heat exchanger, Proc. 6th Int. Conf. Heat Exch. Fouling Clean, Kloster Irsee, Germany, 2005. [16] X.L. Duan, X.Y. Wang, G. Wang, Y.Z. Chen, X.Q. Qiu, Experimental study on the influence of ultrasonic vibration on heat transfer and pressure drop in heat exchanger tubes, Petro-Chem. Equip. 33(1) (2004) 1–4. [17] U. Kurbanov, K. Melkumov, Use of ultrasound for intensification of heat transfer process in heat exchangers, Proc. Int. Cong. Refrig. Vol. 4, Washington DC, USA, 2003, pp. 1–5. [18] A. Monnot, P. Boldo, N. Gondrexon, A. Bontemps, Enhancement of cooling rate by means of high frequency ultrasound, Heat Trans. Eng. 28–1 (2007) 3–8. [19] N. Gondrexon, Y. Rousselet, M. Legay, P. Boldo, S. Le Person, A. Bontemps, Intensification of heat transfer process: improvement of shell-and-tube heat exchanger performances by means of ultrasound, Chem. Eng. Process. 49 (2010) 936–942. [20] M. Legay, S. Le Person, N. Gondrexon, P. Boldo, A. Bontemps, Performances of two heat exchangers assisted by ultrasound, Appl. Therm. Eng. 37 (2012) 60– 66. [21] M.H. Entezari, C. Petrier, P. Devidal, Sonochemical degradation of phenol in water: a comparison of classical equipment with a new cylindrical reactor, Ultrason. Sonochem. 10 (2003) 103–108. [22] P. Vaxelaire, Modular unit for a tubular ultrasonic reactor, US005384508A, January 24, 1995. [23] M. Nakagawa, Analyses of acoustic streaming generated by four ultrasonic vibrators in a vessel, Jpn. J. Appl. Phys. 43(5B) (2004) 2847–2851. [24] J.H. Jeong, Y.C. Kwon, Effects of ultrasonic vibration on subcooled pool boiling critical heat flux, Heat Mass Trans. 42 (2006) 1155–1161.
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Ultrasonics Sonochemistry journal homepage: www.elsevier.com/locate/ultson
Improvement of heat transfer by means of ultrasound: Application to a double-tube heat exchanger M. Legay a, B. Simony a, P. Boldo b, N. Gondrexon a,c,⇑, S. Le Person d, A. Bontemps d a
LEPMI, UMR 5279, CNRS – Grenoble INP – Université de Savoie, Université Joseph Fourier BP 75, 38402 Saint Martin d’Hères, France EDYTEM, UMR 5204, Campus scientifique – Université de Savoie, 73376 Le Bourget du Lac Cedex, France c Laboratoire Rhéologie et Procédés, UMR 5520, CNRS – Université Joseph Fourier Grenoble I, Grenoble-INP BP 53, 38041 Grenoble Cedex 9, France d LEGI, UMR 5519, Domaine Universitaire BP 53, 38041 Grenoble Cedex 9, France b
a r t i c l e
i n f o
Article history: Received 18 October 2011 Received in revised form 3 April 2012 Accepted 3 April 2012 Available online 11 April 2012 Keywords: Ultrasound Heat transfer Heat exchanger Enhancement
a b s t r a c t A new kind of ultrasonically-assisted heat exchanger has been designed, built and studied. It can be seen as a vibrating heat exchanger. A comprehensive description of the overall experimental set-up is provided, i.e. of the test rig and the acquisition system. Data acquisition and processing are explained step-by-step with a detailed example of graph obtained and how, from these experimental data, energy balance is calculated on the heat exchanger. It is demonstrated that ultrasound can be used efficiently as a heat transfer enhancement technique, even in such complex systems as heat exchangers. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction, literature review It is necessary to go back to the 60s to find the first reported studies where ultrasound was used as an active heat transfer enhancement method [1]. At first, it used to concern only convection heat transfer; but results obtained did not seem promising enough to continue investigations on this subject [2,3]. So, the use of ultrasound was then extended to phase change phenomena, successfully gaining in speed during melting for example [4,5]. Boiling heat transfer is another important research field [6–9], where ultrasonic vibrations promote bubbles formation and therefore delay the onset of the critical heat flux. More recently, investigations began to head toward fouling reduction for heat exchangers [10,11], whereas convection is becoming again a subject of interest [12–14]. An undeniable advantage of ultrasound is its cumulative effects leading to several improvements on a single system. Heat exchangers are perfectly suited examples to illustrate this doubly favorable factor, with both heat transfer enhancement and cleaning possibilities [15].
⇑ Corresponding author at: Laboratoire Rhéologie et Procédés, UMR 5520, CNRS – Université Joseph Fourier Grenoble I, Grenoble-INP BP 53, 38041 Grenoble Cedex 9, France. Tel.: +33 4 76 82 65 90; fax: +33 4 76 82 67 77. E-mail addresses: [email protected] (M. Legay), [email protected] (B. Simony), [email protected] (P. Boldo), [email protected] (N. Gondrexon), [email protected] (S. Le Person), [email protected] (A. Bontemps). 1350-4177/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.ultsonch.2012.04.001
However, only a few authors have studied vibrating heat exchangers although very promising results were reported [16,17]. These reasons already led us to develop and study three different heat exchanger configurations to observe the influence of ultrasound in such systems. All of them include two fluids, contrary to most studies in the literature where only one fluid is involved. The first system of interest was a sonochemical reactor containing hot water at rest into which was inserted a copper coil [18]. In this coil was flowing cold water to chill out the reactor content, with and without ultrasound and at frequencies varying from 20 kHz to 1.6 MHz. It was found that the heat transfer coefficient could be increased up to two times, resulting in the enhancement of the cooling rate of the hot water volume. The second system designed was a shell-and-tube heat exchanger where both hot and cold fluids were flowing [19]. Using a special device accurately described thereafter (the SonitubeÒ); it was possible to make the heat exchanger structure resonate at 35 kHz. The overall heat transfer coefficient of this shell-and-tube heat exchanger was enhanced up to 260%. Finally, the third configuration of interest is the one presented in this work: a double-tube heat exchanger. A thermal approach on a slightly different geometry has already been presented and improvements are of the same order as for the shelland-tube heat exchanger [20]. Consequently, the new objectives here are to present a full description of the methods and equipment used and to show as simply as possible that ultrasound, due to its well-known induced effects, is also a valuable technique for heat transfer enhancement inside heat exchangers.
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Nomenclature Latin and Greek symbols A area (m2) Cp specific heat (J kg1 K1) D diameter (m) h convection heat transfer coefficient (W m2 K1) k thermal conductivity (W m1 K1) L length (m) _ m mass flow rate (kg s1) P power (W) q, Q heat flow rate (W) R thermal resistance (K W1) t time (s) T temperature (K) U overall heat transfer coefficient (W m2 K1) U diameter of the SonitubeÒ (m)
2. Description of the system 2.1. Vibrating heat exchanger The vibrating heat exchanger of interest is a double-pipe configuration, i.e. it is made of two concentric straight pipes inserted one into the other. The largest one, external, is a device well-known to sonochemists: the SonitubeÒ (model SM35) [21]. It can be put into vibration at 35 kHz by a piezoelectric transmitter fixed at its center. Besides, its shape was designed with the aim to make it resonate and communicate powerful ultrasonic vibrations to the fluid flowing inside [22]. Indeed, this device was first developed for industrial applications in order to treat a continuous flow of solution by power ultrasound. In this work, it was adapted to build a double pipe heat exchanger by including a straight pipe into it, as schematically shown in Fig. 1. The version of the so defined sono-exchanger used in this work is a modified one compared to that reported for the first time in [20]. The diameter of the internal pipe was reduced in order to ensure a better stability of the generator that supplies ultrasonic power to the system. Two non-vibrating elements were added at each extremity of the vibrating tube to distribute and to collect the fluid at the inlet and outlet of the annular gap. Hot water always flows into the central pipe whereas cold water always flows throughout the annular space. However, the hydrodynamic configuration can be either parallel-flow or counter-flow. The central pipe has a 7 mm internal diameter and is 0.5 mm thick whereas the vibrating tube is 20 mm in internal diameter. The vibrating length is 250 mm and the total length, where heat exchange takes place, is 340 mm. 2.2. Test rig In order to operate the heat exchanger, it was necessary to build all the experimental setup surrounding it (piping, heater, acquisi-
DT
temperature difference (K)
Subscripts amb ambient c cold side, cold water env environment exc exchanged ext external side h hot side, hot water in at the inlet int internal side lm log-mean out at the outlet US ultrasound, in the presence of ultrasound
tion system, etc.). Fig. 2 sketches a simplified version of the global configuration used to run and analyze performances of this vibrating heat exchanger. Compared to the one presented in [20], this experimental setup involves modifications. A volumetric pump and a buffer tank were added on the cold fluid circuit in order to avoid water flow rate fluctuations as it already existed on the hot fluid circuit. This setup is composed of two water circuits: an opened one for cold water and a closed one for hot water. Cold water directly comes from the domestic water supply at a temperature of 15 °C; part of it is stored into the buffer container to ensure a sufficient supply to the pump inlet. Then, cold water flows into the heat exchanger, is heated up and eventually rejected to the sewer. The circuit is almost the same for the hot water except that, once the tank has been filled, water is not rejected and flows in a loop circuit. Therefore, it is continuously re-heated up to 37 °C in the tank to ensure heat exchange with cold water in the heat exchanger. 2.3. Acquisition system Water flow rates are measured by McMillan turbine flowmeters and temperatures are measured by Pt 100 probes (RTD sensors – Resistance Temperature Detector). These apparatus are positioned as indicated in Fig. 2, i.e. the flowmeters are set after the pumps and the temperature sensors are at the inlets and outlets of the heat exchanger. Two other temperature sensors (only useful as indicators but not for calculations) are set in the hot water tank and in the surrounding air. These sensors are connected to a data acquisition module ADAM 4015 whereas the flowmeters are connected to a data acquisition module ADAM 4018. A third and last module is an ADAM 4561 and it serves as a link to the computer to convert RS485 signals to USB. A VBA macro acquires and processes the data in real time.
Fig. 1. Sketch and dimensions of the vibrating double pipe heat exchanger: dimensions in millimetres.
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Fig. 2. Configuration of the global system: simplified schematic diagram.
pipe is set to 0 L/min and only cold water is flowing. If a basic energy balance is performed onto the system working in steady state, it will be found that the ultrasonic power delivered P US is thermally recovered by the cold fluid flowing in the annulus as written in Eq. (1):
_ c CpDT c PUS ¼ m
Fig. 3. Acquisition system setup.
Results are displayed and stored in a spreadsheet file. This acquisition system is sketched in Fig. 3. The VBA macro is set to collect data: the hot and cold fluid flow rates and the four temperatures at the inlets and outlets of the heat exchanger, at regular interval (usually each 2 or 3 s). These data are stored and processed in a spreadsheet document file. Hot and cold heat flow rates, temperature differences and overall heat transfer coefficients are therefore computed and displayed in real time. The uncertainty measurements were estimated at a maximum of 3.1% for the flow rates (according to the measurements and to the flowmeters supplier’s handbook) and at 0.2% for the temperatures.
ð1Þ
PUS is the ultrasonic power dissipated into heat in the cold fluid, computed by multiplying its mass flow rate by the specific heat of the water Cp and by the temperature difference DT c between the inlet and the outlet of the heat exchanger. Fig. 4 shows the evolution of the ultrasonic power transmitted as a function of cold water flow rate in the annulus. Average values obtained from 3 different measurements are plotted on this graph. Experiments were even repeated with the central pipe completely empty of water (filled with air to change the acoustic impedance) and results were the same. This estimation of the ultrasonic power dissipated into heat in the annulus is necessary to fulfill the energy balances when both fluids are flowing. Except for very low water flow rates in the annulus (e.g. 0.2 L/ min, PUS 50 W), the ultrasonic power transmitted is quite stable:
3. Methodology for results analysis 3.1. Estimation of the ultrasonic power transmitted Before beginning the heat transfer experimentations, it is necessary to estimate the ultrasonic power delivered to the fluid. It is assumed that all the mechanical energy of the waves is ultimately dissipated into heat at the annulus side (in the cold fluid in Fig. 1). It is therefore possible to evaluate the amount of ultrasonic power transmitted to this fluid, by calorimetric measurement in a dynamic mode. To do so, the hot water flow rate inside the central
Fig. 4. Calorimetric measurements: average ultrasonic power transmitted to the cold fluid in the annulus versus its flow rate.
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around 70–75 W. It means that the water flow rate has very little influence on the energy dissipated in the heat exchanger for this geometry. The lower ultrasonic power transmitted, at low water flow rates, is explained in part by automatic adjustment of the power delivered by the Sonitube’s generator. There is indeed a minimal fluid velocity for good frequency resonance matching between the transducer, the vibrating structure and the fluid flow as explained in the Sonitube supplier’s handbook. The generator output power was never modified manually during the tests and was kept constant at its minimum value, i.e. 100 W at the most. In fact, the generator can deliver up to 400 W but it is manually set to 50% amplitude and besides, a de-booster is used after the piezoelectric transducer, reducing again the amplitude by half before entering the structure (sonotrode). It was necessary to undertake such settings in order to have the best heat exchanger energetic efficiency as possible. To reduce more the ultrasonic power was unfortunately impossible since otherwise, the generator power output would not be sufficient to start the vibration of the whole structure. 3.2. Typical interpretation of collected data In addition to the computing and recording of collected values, the software is also capable of plotting in real time the evolution of the four temperatures (2 inlets and 2 outlets) versus time. It is therefore possible to see immediately on a graph when the heat transfer process reaches its steady state (usually only a few minutes). An example of typical graph obtained is proposed in Fig. 5. At initial time t = 0 s when the acquisition program is launched, the steady state of the heat transfer process under silent conditions has already been reached. That is why the four temperatures are stable until t = t1, i.e. they are not evolving in time if no change (e.g. flow rate or energy supply) is applied to the system. The temperature difference is greater for the cold water flow since its flow rate is lower than the hot water one and the heat flux exchanged is equal for both fluids (see Section 3.3 for equations and explanation). At time t1 40 s, the ultrasound generator is turned on and the heat exchanger structure is put into vibration. The two inlet temperatures of hot and cold water (‘‘cold in’’ and ‘‘hot in’’ in Fig. 5)
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are not affected by ultrasound since the sensors are set just before the inlets of the heat exchanger, so these flows of water continue to arrive at the same temperature. Conversely, the two outlet temperatures (‘‘cold out’’ and ‘‘hot out’’ in Fig. 5) are changed to a great extent: the cold one becomes hotter whereas in the meantime, the hot one is slightly lowered. This clearly shows an improvement of the heat exchange rate. The increase of the cold fluid outlet temperature is notably more important than the decrease of the hot fluid outlet temperature. This is explained by the ultrasonic power dissipated into heat at the cold side, increasing the temperature difference. It is also due to the lower mass flow rate (ratio 0.5/ 1.6): to collect the same amount of energy, a larger temperature difference is needed (see for instance Eq. (1)). At time t2 135 s, ultrasound is turned off and the temperatures at the outlets regain their initial values without ultrasound. 3.3. Energy balance In order to analyze the heat exchanger performances, energy balances with and without ultrasound are performed. Fig. 6 shows a schematic representation of the system considered, in a parallelflow configuration. Without ultrasound, the energy balance is obvious (Eq. (2)): it says that the power transferred by the hot water flow corresponds to the power received by the cold water flow (assuming no heat transfer to the environment). It is also called the exchanged heat flow rate qexc .
qc ¼ qh ¼ qexc
ð2Þ
Eq. (2) describes what happens between t0 and t1 in the graph Fig. 5. Another term, corresponding to the heat transfer rate to the environment qenv , is sometimes added. Therefore, the heat exchanger cannot be considered as perfectly adiabatic, although it is very well insulated. qenv is always negligible, i.e. inferior to 10% of qexc . Taking qenv into account, Eq. (2) then becomes Eq. (3).
qc þ qenv ¼ qh
ð3Þ
qenv is generally negative (with respect to its corresponding arrow in Fig. 6, i.e. it will contribute to the heating of the cold water flow) since T amb > T c , which is usually the case in this work. Indeed, T amb was between 20 and 25 °C during all the series of experiments whereas the cold water temperature is around 15 °C in average. With ultrasound, the energy balance is written as in Eq. (4):
qc þ qenv ¼ qh þ PUS
ð4Þ
The ultrasonic power transmitted PUS is a positive quantity (because entering the system) estimated previously for each flow rate as explained in Section 3.1. qc and qh can be calculated respectively with Eq. (5) and Eq. (6), with or without ultrasound:
_ c CpðT c;out T c;in Þ qc ¼ m _ h CpðT h;in T h;out Þ qh ¼ m
ð5Þ ð6Þ
In the presence of ultrasound (or also when there is heat flow to or from the environment), qc and qh are not equal any more. Therefore, it is qh that is referred to as qexc in order to avoid taking into account the term corresponding to ultrasonic energy dissipation.
Fig. 5. Evolution of the four temperatures versus running time, without and with ultrasound. Cold water flow rate in the annulus: 0.5 L/min, hot water flow rate in the tube: 1.6 L/min.
Fig. 6. Energy balance on the system considered.
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4. Results and discussion 4.1. Chart bar presentation of the energy balance From the energy balance of Eq. (4), it is possible to draw chart bar diagrams to distinguish better the enhancement brought by ultrasonic vibrations. The amounts reported in the bars are the instantaneous energy transfer rates obtained when the heat exchanger is working in steady state regime. Fig. 7 shows an example of energy balance obtained without and with ultrasound, for a cold water flow rate equal to 0.5 L/min and a hot water flow rate equal to 1.6 L/min, in a parallel flow configuration. In this example, there is no heat flow rate to the environment without ultrasound. The heat flow rate exchanged is equal to 128 W: the two bars are precisely at the same level, which exactly corresponds to Eq. (2). With ultrasound, the four quantities of the energy balance have been plotted side by side. It is then possible to virtually superpose the bars in order to correspond exactly to the energy balance of Eq. (4) and to obtain once again two bars up to the same level (each side of Eq. (4)). Thus, the energy balance is fulfilled: 273 W at each side, meaning some enhancement owing to the effects of vibrations. By subtraction, one can easily quantify the improvements: bringing 67 W of ultrasonic power (computed by calorimetric measurement as explained in Section 3.1), the exchanged heat flow rate (qexc ¼ qh ) is increased by 78 W (206– 128) and qc is increased by approximately the same amount plus the ultrasonic power dissipated (also ±qenv in reality, i.e. 130 W total increase in this example). In the presence of ultrasound, the heat flow rate to the environment qenv is positive, which means that there are heat losses (same direction as the arrow in Fig. 6). It can also be due to an over-estimation of the ultrasonic power transmitted that can sometimes fluctuate around the value estimated in the calorimetric section. Nevertheless, qenv is still considered negligible for being inferior to 10% of the heat flow rate exchanged. Fig. 8 shows the same kind of diagram for the same water flow rates but in a counter-current configuration. The only parameter that has been changed compared to the previous example in Fig. 7 is the direction of hot water circulation. Without ultrasound, heat flow rates are higher than in parallelflow configuration, which is a typical and well-known observation due to the more elevated temperature difference between the fluids all along the heat exchanger. The heat flow rates at the hot and
Fig. 7. Example of energy balance in a chart bar form: cold water flow rate 0.5 L/ min, hot water flow rate 1.6 L/min, parallel flow configuration.
Fig. 8. Chart bar presentation of the energy balance: cold water flow rate 0.5 L/min, hot water flow rate 1.6 L/min, counter-flow configuration.
cold sides are almost equal although there is a negligible heat flow rate from the environment (see Eq.(3)): 5 W, contributing to heating the cold fluid (direction opposite to the arrow of Fig. 6). With ultrasound, improvements in qh and qc , 63 and 136 W, are also relevant in this counter-current configuration since the ultrasonic power delivered is equal to 67 W. The heat flow rate to the environment is still negative (heating the cold water flow) and negligible compared to qexc . From Figs. 7 and 8, it is possible to conclude that heat transfer enhancement is not only due to the ultrasonic power dissipated into heat at the cold side. Ultrasound has side-effects enhancing heat transfer, discussed in Section 4.2. 4.2. Effects of ultrasound on the overall heat transfer coefficient The attention is now turned to the analysis of the overall heat transfer coefficient U, to try to find explanations to the improvements. Experimental values of U are calculated with Eq. (7):
U¼
qexc ADT lm
ð7Þ
Fig. 9. Overall heat transfer coefficient versus cold water flow rate in the annulus: influence of flow configuration (hot water flow rate fixed: 1.6 L/min).
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where DT lm is the log-mean temperature difference and A is the area across which heat transfer occurs. Remembering also that qexc is taken equal to qh to avoid an overestimation of the improvements (the hot fluid is in the central pipe, not directly subjected to ultrasound or to heat transfer to/from the environment). This coefficient gives a global view of heat exchanger performances, which is why it is of great interest. Fig. 9 presents the evolution of U for four different configurations: parallel and counter-flow, both with and without ultrasound. Four important observations have to be made about Fig. 9:
the annulus for the following interpretation. In the form of a theoretical equation, the overall heat transfer coefficient U of a doubletube heat exchanger can be written as the sum of three thermal resistances, as in Eq. (8):
– There is no notable difference between the overall heat transfer coefficients in parallel-flow or counter-flow configurations for the same flow rates considered. This is easily explained: no change in flow rates imply no change in convection coefficients and therefore no modification of the overall heat transfer coefficient (see also Eq. (8) thereafter). The highest heat exchange flow rates usually obtained in counter-flow configurations are only due to the higher temperature difference across the heat exchanger. – The improvements in the presence of ultrasound are noticeably the same for both configurations. Indeed, there is no apparent reason why intensification would be greater if water flows in one direction or another, if the flow rates and the other parameters do not change. – Ultrasonic intensification is more important for low cold water flow rates. A possible interpretation of this observation is given by Fig. 10. – Conversely, in a transition/turbulent flow (P3 L/min), the ultrasonic improvements are reduced. Explanation following Fig. 10 gives a potential reason.
The three terms on the right side of the equation correspond to each thermal resistance respectively named R1, R2 and R3. D is the diameter of the tube, L is its length, k is the thermal conductivity of the metal and h is the convective heat transfer coefficient. Fig. 10 shows a zoom of the internal pipe thickness, in the longitudinal cross-section, to visualize each of these thermal resistances. For this schematic drawing, a laminar flow has been chosen in the annulus side and a turbulent flow in the pipe side. This corresponds to the configuration where the highest improvements were observed [19], and to the previous examples (Figs. 5 and 7–9). There are three zones represented in Fig. 10, corresponding to the three thermal resistances:
In the heat exchanger studied, ultrasound is applied to the shell (external tube). Therefore, the effects are likely to be much more intense in the shell side (annulus) than inside the internal tube. This is supported in Section 3.1 by the fact that no change were observed when measuring the ultrasonic power transmitted with the central pipe either empty or filled with water at rest. If water in the central pipe was affected by ultrasound (e.g. absorbing part of the ultrasonic power delivered), PUS would probably be modified. This is the reason why effects are assumed negligible elsewhere than in
1 ¼ R1 þ R2 þ R3 UA
ð8Þ
Expressing each resistance term with geometrical and physical parameters, Eq. (8) becomes Eq. (9):
1 1 lnðDext =Dint Þ 1 þ ¼ þ UA h1 A1 2pkL h 3 A3
ð9Þ
– The annulus side, where the cold water flow is laminar and the thermal resistance (R1) is therefore very important (without ultrasound) due to the low velocity of water. – The pipe thickness (0.5 mm, stainless steel), where conduction heat transfer takes place. The thermal resistance (R2) is always negligible compared to the other ones. – The internal pipe side, where the liquid flow is turbulent and the thermal resistance (R3) is consequently assumed lower than the one in the annulus (but still higher than the thermal conduction resistance). It can be seen on Fig. 10 that ultrasound, transmitted in the annulus, has the effect to decrease R1 to R1US by enhancing the convection heat transfer coefficient h1. Without ultrasound, R1 is the limiting thermal resistance, i.e. the one that hampers heat transfer the most. By decreasing it, the overall heat transfer coefficient is increased and heat exchange process is enhanced globally.
Fig. 10. Illustration of ultrasound effects: heat transfer enhancement inside the heat exchanger.
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The thermal conduction resistance in the pipe thickness R2 and the convection resistance in the internal pipe R3 are assumed unchanged by the ultrasound. The common ultrasonically-induced phenomena are usually held responsible for these improvements. Acoustic cavitation, and especially bubbles oscillation and powerful implosion, creates micro-turbulence at the solid–liquid interface and breaks the dynamic boundary layer. The fluid is therefore mixed and the convection phenomenon is strongly intensified. Vibrations of the walls and inside the fluid are also likely to have some non-negligible influence on the fluid stirring. This would also explain why when the flow is already turbulent in the annular space, improvements are less pronounced (right side of Fig. 9). In this work, acoustic streaming, usually reported to have great influence on heat transfer [23,24], is likely to be negligible since fluids are already flowing due to the pumps. 5. Conclusion An ultrasonically-vibrating tube-in-tube heat exchanger has been studied in this work. This new type of heat exchanger was inserted within a test rig that has been thoroughly detailed in this paper. In particular, the data acquisition system specifically developed for this work has been reported. It enables to follow the evolution versus running time of the temperatures at the inlet and the outlet of the heat exchanger for both fluids. The simple observation of the graph obtained is a qualitative way to illustrate the improvement of heat transfer in the presence of ultrasound. In a second step, an energy balance is performed based upon data collected in steady-state operation of the heat exchanger. This energy balance is expressed through classical thermal equations, taking into account the ultrasonic power provided to the fluid in the annular space, estimated by calorimetric measurements. Bar graphs were used to illustrate values of this energy balance and to demonstrate the enhancement in the presence of ultrasound. Finally, overall heat transfer coefficients were classically determined with and without ultrasound. It has been shown that, whatever the heat exchanger flow configurations, the overall heat transfer coefficients in the presence of ultrasound are higher than under silent conditions. It was assumed that the conduction thermal resistance through the thickness of the internal pipe and that the convection thermal resistance inside the internal pipe are not affected by ultrasonic waves. It was therefore concluded that only the convection thermal resistance in the annular space was decreased, leading to an enhanced exchanged heat flow rate. It is considered that physical effects induced by cavitation bubbles implosion as well as vibrations of the walls result in a disturbance of the dynamic boundary layer. To conclude, ultrasound is shown here to be an interesting way to improve heat exchanger performances. Further studies would consist in investigating the potential of ultrasound for fouling reduction in heat exchangers.
Acknowledgement This work was supported by the Association Institutes Carnot, Energies du futur. References [1] M. Legay, N. Gondrexon, S. Le Person, P. Boldo, A. Bontemps, Enhancement of heat transfer by ultrasound: review and recent advances, Int. J. Chem. Eng. (2011) 1–17, doi:http://dx.doi.org/10.1155/2011/670108. [2] A.E. Bergles, P.H. Newell Jr., The influence of ultrasonic vibrations on heat transfer to water flowing in annuli, Int. J. Heat Mass Trans. 8 (1965) 1273– 1280. [3] M.B. Larson, A study of the effects of ultrasonic vibrations on convective heat transfer in liquids, Ph.D. Dissertation, Stanford University, Mic, 1961, pp. 61– 1235. [4] H.V. Fairbanks, Influence of ultrasound upon heat transfer systems, Ultrason. Symp. (1979) 384–389, http://dx.doi.org/10.1109/ULTSYM.1979.197226. [5] Y.K. Oh, S.H. Park, Y.I. Cho, A study of the effect of ultrasonic vibrations on phase-change heat transfer, Int. J. Heat Mass Trans. 45 (2002) 4631–4641. [6] S.W. Wong, W.Y. Chon, Effects of ultrasonic vibrations on heat transfer to liquids by natural convection and by boiling, AIChE J. 15–2 (1969) 281–288. [7] K.A. Park, A.E. Bergles, Ultrasonic enhancement of saturated and subcooled pool boiling, Int. J. Heat Mass Trans. 31–33 (1988) 664–667. [8] S. Bonekamp, K. Bier, Influence of ultrasound on pool boiling heat transfer to mixtures of the refrigerants R23 and R134A, Int. J. Refrig. 20–8 (1997) 606– 615. [9] F. Baffigi, C. Bartoli, Heat transfer enhancement from a circular cylinder to distilled water by ultrasonic waves in subcooled boiling conditions, Proc. ITP, October 4–9, Volterra, Italy, 2009. [10] T.R. Bott, Biofouling control with ultrasound, Heat Trans. Eng. 21 (2000) 43–49. [11] T.R. Bott, L. Tianqing, Ultrasound enhancement of biocide efficiency, Ultrason. Sonochem. 11 (2004) 323–326. [12] M.K. Aktas, B. Farouk, Y. Lin, Heat transfer enhancement by acoustic streaming in an enclosure, J. Heat Trans. 127 (2005) 1313–1321. [13] S. Hyun, D.R. Lee, B.G. Loh, Investigation of convective heat transfer augmentation using acoustic streaming generated by ultrasonic vibrations, Int. J. Heat Mass Trans. 48 (2005) 703–718. [14] S. Nomura, M. Nakagawa, S. Mukasa, H. Toyota, K. Murakami, R. Kobayashi, Ultrasonic heat transfer enhancement with obstacle in front of heating surface, Jpn. J. Appl. Phys. 44(6B) (2005) 4674–4677. [15] W. Benzinger, U. Schygulla, M. Jäger, K. Schubert, Anti-fouling investigations with ultrasound in a microstructured heat exchanger, Proc. 6th Int. Conf. Heat Exch. Fouling Clean, Kloster Irsee, Germany, 2005. [16] X.L. Duan, X.Y. Wang, G. Wang, Y.Z. Chen, X.Q. Qiu, Experimental study on the influence of ultrasonic vibration on heat transfer and pressure drop in heat exchanger tubes, Petro-Chem. Equip. 33(1) (2004) 1–4. [17] U. Kurbanov, K. Melkumov, Use of ultrasound for intensification of heat transfer process in heat exchangers, Proc. Int. Cong. Refrig. Vol. 4, Washington DC, USA, 2003, pp. 1–5. [18] A. Monnot, P. Boldo, N. Gondrexon, A. Bontemps, Enhancement of cooling rate by means of high frequency ultrasound, Heat Trans. Eng. 28–1 (2007) 3–8. [19] N. Gondrexon, Y. Rousselet, M. Legay, P. Boldo, S. Le Person, A. Bontemps, Intensification of heat transfer process: improvement of shell-and-tube heat exchanger performances by means of ultrasound, Chem. Eng. Process. 49 (2010) 936–942. [20] M. Legay, S. Le Person, N. Gondrexon, P. Boldo, A. Bontemps, Performances of two heat exchangers assisted by ultrasound, Appl. Therm. Eng. 37 (2012) 60– 66. [21] M.H. Entezari, C. Petrier, P. Devidal, Sonochemical degradation of phenol in water: a comparison of classical equipment with a new cylindrical reactor, Ultrason. Sonochem. 10 (2003) 103–108. [22] P. Vaxelaire, Modular unit for a tubular ultrasonic reactor, US005384508A, January 24, 1995. [23] M. Nakagawa, Analyses of acoustic streaming generated by four ultrasonic vibrators in a vessel, Jpn. J. Appl. Phys. 43(5B) (2004) 2847–2851. [24] J.H. Jeong, Y.C. Kwon, Effects of ultrasonic vibration on subcooled pool boiling critical heat flux, Heat Mass Trans. 42 (2006) 1155–1161.