This paper summarizes some applications of ultrasonic vibrations regarding heat transfer enhancement techniques. Research literature is reviewed, with special attention to examples for which ultrasonic technology was used alongside a conventional heat transfer process in order to enhance it. In several industrial applications, the use of ultrasound is often a way to increase productivity in the process itself, but also to take advantage of various subsequent phenomena. The relevant example brought forward here concerns heat exchangers, where it was found that ultrasound not only increases heat transfer rates, but might also be a solution to fouling reduction.
In engineering applications, ultrasound is helpfully used to improve systems efficiencies. Intensifying chemical reactions, drying, welding, and cleaning are among the various possible applications of ultrasonic waves [
It appears that researches undertaken in the past concerned basic systems, usually with a single fluid, such as heating rods or walls in a volume of water subjected to ultrasonic vibrations. The tendency goes toward systems getting more complicated (e.g., cooling of tiny components, vibrating structures for heat exchangers) and models becoming more accurate with powerful numerical simulations for example.
The objectives of this paper are to provide scientific and historical backgrounds to the future studies concerning heat transfer enhancement by ultrasonic vibrations and to bring forward the evolution of this domain with several examples of applications. The first part describes an overview of ultrasound, induced phenomena, and how they positively influence heat transfer processes. Then, examples drawn from various fields of interest are analysed (thermal engineering, food industry, experimental and numerical simulations). Emphasis is made on the best improvements and results obtained. Finally, recent adaptation of ultrasonic technologies to heat exchanger devices is discussed thoroughly, with examples drawn from new patents and current laboratory work.
Acoustic waves of which frequencies are higher than the upper limit of the human hearing range, usually around 16 or 20 kHz, are called ultrasound. These waves are often classified according to their frequency or power.
Between 20 and about 100 kHz, waves are defined as “low frequency ultrasound” or “power ultrasound”. Indeed, it is usually transferred at a high power level (a few tens of Watts), and therefore, ultrasound is able to modify the medium where it propagates. Power ultrasound can disrupt a fluid bulk to create cavitation or acoustic streaming, two phenomena with powerful macroscopic effects for heat transfer enhancement. Therefore, power ultrasound finds uses in various processes like cleaning, plastic welding, sonochemistry [
Further in the frequency spectrum, above 1 MHz, is found “low power ultrasound” (usually less than 10 W), at a “very high frequency” which does not affect the medium of propagation. Consequently, it is especially used for medical diagnosis or nondestructive material control, and references regarding heat transfer enhancement are very scarce in the literature.
In the intermediate range 100 kHz–1 MHz, “high frequency ultrasound” is found. It is less used than power ultrasound to promote heat transfer. Figure
Utilizations of ultrasound according to frequency and power.
Many phenomena may ensue from propagation of an ultrasonic wave into a fluid and more particularly into a liquid medium. Two of them, of major importance for heat transfer enhancement, are acoustic cavitation and acoustic streaming. There exist other subsequent effects such as heating of the medium due to dissipation of the mechanical energy. This phenomenon is used for the determination of the ultrasonic energy supplied to the medium in an ultrasonic reactor, well-known as the calorimetric method [
Four effects resulting from ultrasound propagation in a liquid.
These phenomena have always been a subject of interest since their discovery, and even though research is still ongoing, some comprehensive descriptions have been made by several authors and are frequently updated [
Acoustic streaming can be considered as a well-known phenomenon since its comprehensive mathematical description by Lighthill in 1978 [
Acoustic streaming—enhancement of convection heat transfer.
Fand and Kave [
Acoustic streaming (forced air current) was created in the air above a vibrating beam [
Acoustic streaming is also a factor that reduces the melting time of paraffin [
A type of configuration often studied is heat transfer occurring in a channel made by two plates or beams at different temperatures with vibrations applied either to the fluid between or to one of the walls [
The typical order of magnitude of acoustic streaming velocity is usually a few centimetres per second (between 1 and 100 cm s−1) [
Acoustic cavitation is the major phenomenon that may arise from the propagation of ultrasonic waves into a liquid. Many authors have described cavitation process thoroughly but not always appearing in an oscillating pressure field, in which particular case is called acoustic cavitation [
There are many other ways to create cavitation into a liquid, for instance, hydrodynamic cavitation using micro-channels which can also promote cooling heat transfer [
There exist two types of acoustic cavitation: stable and transient [
Explanation of heat transfer enhancement by acoustic cavitation.
Usually the bubble implosion is assumed to be of the order of the microsecond, and the bubble size is about 10−4 m (but also depending on frequency) [
It is necessary to go back to the 60s to find the first reported studies dealing with heat transfer intensification involving ultrasonic vibrations. These very pioneer studies (see also Section
Evolution of the number of published papers per decade dealing with heat transfer enhancement by ultrasound.
Very few works were published in the 70s–80s but an important increase has taken place since the 90s. According to this tendency, one can expect that in the forthcoming years, this subject is likely to know a substantial development.
Among the three heat transfer modes, conduction and radiation assisted by ultrasound are the less studied. Strangely, only a few authors have investigated them although promising results were already reported in 1979 by Fairbanks [
Power ultrasound is a method to reduce the size of ice crystals on the frozen products and gain in quality [
The freezing temperature of supercooled water can also be controlled by ultrasonic vibrations to make ice slurry, a solid-liquid mixture very interesting to store and transport cold thermal energy. The probability of phase change is increased with the total number of cavitation bubbles, acting as nuclei for solidification inception [
Various uses of ultrasound to promote phase change heat transfer.
Reference | Description of the study | Frequency, power, intensity | Best and/or interesting result obtained |
---|---|---|---|
Fairbanks [ | Radiation (Sun and infrared) into water, conduction into metal, melting heterogeneous system | 50 kHz, 61 W (radiation); 20 kHz, 250 W (melting); 20 kHz, 75 W (conduction) | Radiation: double heat transfer rate, conduction: 3.55 times thermal conductivity, melting rate doubled |
Inada et al. [ | Experimental, phase change from supercooled water to ice, acoustic cavitation, pure water and tap water | 28 kHz, 0–6.5 kW m−2 | Important decrease of supercooling with ultrasound for both types of water |
Oh et al. [ | Melting of paraffin in a tank with constant heat flux, acoustic streaming, cavitation, experimental and modelling study | 40 kHz, 70–340 W | Melting time 72 min with 340 W ultrasonic power instead of 275 min without ultrasound |
Zhang et al. [ | Experimental study, probability of water phase change with number of bubble nuclei, cavitation, square vessel, transducer at the bottom | 39 kHz, 4.4 kW m−2 | Probability of phase change increased with number of bubble nuclei and pressure amplitude |
Boiling heat transfer in the presence of an ultrasonic field is described apart for being a very active research field. Ultrasound allows improvement of boiling heat transfer almost systematically. The first bubbles appearing in the nucleation sites are swept away by the vibrations, and the apparition of film boiling is therefore delayed so that higher heat fluxes are reached (see Figure
Enhancement of boiling heat transfer by ultrasonic vibrations.
According to several authors, this is still due to acoustic cavitation, which helps the creation and growth of the bubbles, whereas their oscillations enable to create micro-streaming and local agitation near the surfaces to sweep them away [
Heat transfer enhancement of saturated pool nucleate boiling was studied using a combined method: ultrasonic vibrations and glass beads (49
It was reported several times that the distributions of the sound pressure and of the local heat transfer enhancement were in phase [
The critical heat flux of subcooled boiling in water in the presence of ultrasound is influenced by several parameters [
Table
Summary table of boiling heat transfer studies.
Reference | Description of the study | Frequency, power, intensity | Best and/or interesting result obtained |
---|---|---|---|
Baffigi and Bartoli [ | Experimental, subcooled boiling, horizontal cylinder, cavitation | 40 kHz, 300–500 W | |
Bergles and Newell [ | Horizontal annulus, subcooled boiling, CHF | 70 kHz; 80 kHz, 1.4 W/cm² | 70 kHz, 40% local increase in non-boiling |
Bonekamp and Bier [ | Pool boiling, pure fluids (R23, R134a), and mixtures of both | 42.0 kHz; 69.2 kHz; 84.7 kHz, 4 W | 42 kHz, equimolar mixture, |
Heffington and Glezer [ | Pool boiling enhancement, VIBE mechanism (vibration-induced bubble ejection) | 1.65 MHz | Water/ethanol ~70/30: 425% increase in CHF (600 W cm−2) |
Jeong and Kwon [ | CHF augmentation pool and subcooled boiling, inclination angle | 40 kHz | 87–126% CHF increase for downward facing surface |
Kim et al. [ | Experimental results, natural convection, pool subcooled and saturated boiling, platinum wire, transducer at the bottom, liquid FC-72 | 48 kHz | At least 60% global heat transfer increase (natural convection) |
Kim and Jeong [ | Numerical study, water bath, transducer at the bottom, inclination and subcooled boiling | 40 kHz | see Jeong and Kwon [ |
Kwon et al. [ | CHF enhancement pool boiling, variation of inclination angle and pool temperature, transducer at the bottom | 40 kHz | CHF increased by 110% at pool temperature 95°C, horizontal downward plate |
Park and Bergles [ | Inert, dielectric liquid typical of those used for immersion cooling of microelectronic components (R-113) to cool small diameters stainless steel tubes power supplied | 55 kHz, 75 W, 8000 W m−2 | Saturated pool: 10% increase in burnout heat flux; subcooled pool: 5% increase |
Serizawa et al. [ | Horizontal and vertical surfaces in water and vertical round tube under forced circulation of water. Silver rod at 750–800 K into distilled water (film boiling), ultrasound at the bottom | 28 kHz, 70 W | Natural convection and pool nucleate boiling augmented for higher liquid subcooling. Temperature change periodically with ultrasonic waves. |
Wong and Chon [ | Natural convection and boiling around platinum wire in distilled water and methanol, cavitation, experimental work | 20 kHz; 44 kHz; 108 kHz; 306 kHz, 0–200 W (with amplifier) | 8-fold increase in heat transfer coefficient in natural convection |
Yamashiro et al. [ | Quenching process, horizontal platinum wires in subcooled water | 24 kHz; 44 kHz, 0–280 W | Cooling rate and heat flux increase with cavitation intensity and water subcooling, better effect at 24 kHz |
Zhou and Liu [ | Experimental study, acetone boiling in cubic pool around an horizontal circular tube, acoustic cavitation | ? | Heat transfer increased with water subcooling and cavitation intensity |
Zhou [ | Experimental investigations, copper nanofluid, acoustic cavitation, cubic vessel filled with acetone, horizontal copper tube | ? | Heat transfer in presence of acoustic field increased with nanoparticles concentration, cavitation intensity, fluid subcooling |
Zhou and Liu [ | Experimental investigations, calcium-carbonate nanoparticles in acetone, acoustic cavitation, cubic vessel with horizontal copper tube | ? | Convection and boiling reduced by addition of nanoparticles, but increase with acoustic field intensity |
Zhou et al | Acetone boiling around horizontal copper tube in a cubic vessel, acoustic cavitation effect on boiling heat transfer | ? | Higher heat flux at lower wall temperature with acoustic cavitation |
For being particularly adequate (nonintrusive, nonchemical, etc.), ultrasonic technologies are intensively developing in food industry. Food drying is one of the best examples. If there is a good acoustic match between the transducer and the food material, ultrasonic vibrations can be directly applied to the material to be dried [
Sponge effect during vibration and drying of a porous food product.
Power ultrasound mainly affects the external thermal resistance. If the transducer is not in contact with the material and ultrasound is air-borne, it is reported that high air flow rate may introduce modifications in the acoustic field, decreasing also the acoustic energy transmitted to the medium. Power ultrasound increases the effective moisture diffusivities at low air velocities but the improvement becomes negligible at high air velocities [
With the aim of sterilizing food, the influence of particle size and power input on heat transfer between fluid and food size particles was investigated [
Reported uses of ultrasound in food industry.
Reference | Description of the study | Frequency, power, intensity | Best and/or interesting result obtained |
---|---|---|---|
Cárcel et al | Drying persimmon cylinders, air velocity change, experiments, and mathematical model | 21.8 kHz, 75 W, 154.3 dB | Drying speed increased with ultrasound at low air velocities (<4 m s−1), affecting internal and external thermal resistances |
de la Fuente-Blanco et al. [ | Drying process with direct contact, vibrating plate | 20 kHz, 0–100 W | At 100 W power, after 60 min, sample mass 27% instead of 85% |
Gallego-Juárez et al. [ | Drying process with direct contact, vibrating plate | 20 kHz, 100 W | Final moisture less than 1%, speed increase, and better quality product |
Li and Sun [ | Experimental study: potatoes samples freezing into 50/50% mixture water/ethylene glycol at about −18°C | 25 kHz, 7.34 W; 15.85 W; 25.89 W | Most efficient power: 15.85 W; exposure time: 2 min; during the phase change period |
Mason et al. [ | Review article (food technology) | ||
Sastry et al. [ | Sterilization applications but food particles replaced by metal samples. Effect of size and power input | Power input: 0.139, 0.069 and 0.046 W g−1 of liquid | Convection coefficient approximately doubled in all cases |
Zheng and Sun [ | Review article (food freezing process) |
Convection, like boiling, is one of the most studied modes of heat transfer under the influence of ultrasonic vibrations. Increases in heat transfer coefficients up to 25 times are reported [
Fand and Kave [
Conversely, in Larson's Ph.D. dissertation [
Experiments and numerical results reported by Gould [
Using frequencies below 20 kHz, Komarov and Hirasawa [
At a local scale, in a stationary acoustic field, it was observed that the convection heat transfer coefficient was the highest where the sound pressure was maximal [
Dissolved gas can also have an influence as illustrated in [
The influence of the fluid characteristics has also some importance, as shown in [
More unusual studies have also been undertaken like the influence of nanoparticles combined with acoustic cavitation on convection and boiling [
Two graphs have been plotted in Figures
Increase in convection heat transfer coefficient versus ultrasonic power.
Influence of frequency on the increase of convection heat transfer coefficient.
One can see in Figure
Concerning Figure
An interesting experimental setup is described in [
Nomura et al. [
An interesting and original use of power ultrasound is for wood treatment [
In the domain of ultrasonically improved heat transfer, convection is the most studied area, as illustrated by Figure
Percentage of studies by subject (total: 62 papers from the tables of this document).
This chart was made with all references quoted in the tables except Table
A very important point is the cause of heat transfer enhancement, which is very difficult to determine since many phenomena appear simultaneously during propagation of ultrasound. Figure
Ultrasound effects held responsible for heat transfer enhancement.
According to this statistic chart, acoustic cavitation is the predominant phenomena for heat transfer enhancement. It is followed by acoustic streaming and by local agitation due to oscillations. Other phenomena, such as fouling reduction, hysteresis reduction, change in bubble behaviour, are side effects that could become very important when ultrasound will be used in industrial systems.
Table
Ultrasonic waves and convection heat transfer improvements.
Reference | Description of the study | Frequency, power, intensity | Best and/or interesting result obtained |
---|---|---|---|
Bergles [ | Review article, heat transfer enhancement | ||
Cai et al. [ | Experimental, natural convection, acoustic cavitation, circular heated copper tube in water, brine and sugar water | 18 kHz, 0–250 W | Heat flux from cylinder: 132 W m−2, ultrasonic intensity: 80 W cm−2, enhancement up to 360%. |
Fand and Kave [ | Acoustic streaming, convection heat transfer, heated cylinder | 800 Hz–4800 Hz | 3-fold increase in heat transfer rate |
Gould [ | Acoustic streaming, convection between metal and water or glycerin-water mixtures | ? | Up to 10-fold increase |
Hoshino and Yukawa [ | Experimental investigation, hot and cold cylinders, vertical standing waves, local and global coefficients in degassed water | 28 kHz, 0.1–0.215 W cm−2 | Local coefficient |
Hoshino et al. [ | Free convective heat transfer from a heated wire | 28 kHz | Local coefficient |
Hyun et al. [ | Experiments and CFD simulations of acoustic streaming induced by flexural vibrations of a beam, cooling of a stationary beam above | 28.4 kHz | Temperature drop of 40°C in 4 min, |
Iida et al. [ | Experimental, natural convection heat transfer from a fine cylinder to water, comparison convection coefficient and sound pressure profiles | 28 kHz | Augmentation ratio around 1.6 when Δ |
Komarov and Hirasawa [ | Standing and travelling sound waves in tubes, platinum wire | 0.3–17.2 kHz | |
Lam et al. [ | Experimental study, saturated and air-dried wood cylinders heated in a water bath at 59.8°C with and without ultrasound | 50–55 kHz, | Significant influence of ultrasound on the temperature increase at the centre of cylinders |
Larson [ | Acoustic streaming around a sphere within a cylinder, cavitation, toluene, and water | 20–1000 kHz, | Increase in Nusselt number up to about 4 times, but not sufficient to warrant the technology |
Lee and Loh [ | Acoustic streaming in a gap between heat source and transducer | 30 kHz | Heat transfer rate increased up to 75% |
Lee and Choi [ | Acoustic cavitation into CO2 saturated water | 138 W | Up to 369.5% turbulence intensity enhancement |
Loh et al. [ | Experiments and simulations (CFX4), flexural vibrations of a beam, acoustic streaming in air above (open) to cool a fixed beam | 28.4 kHz | Temperature drop of 40°C in 4 min, streaming velocity up to 2 m s−1 |
Markov et al. [ | Flowing molten metal (~1520°C) in a water-cooled tube | 20 kHz | Heat transfer coefficient as much as doubled |
Nakagawa [ | Experimental and computational results (CFX4), 4 vibrators to control acoustic streaming in a vessel containing silicon oil | 1 MHz | Maximum streaming velocity measured: 0.07 m s−1, jet position modified |
Nakayama and Kano [ | Experiments, cylindrical glass vessel, distilled water, with or without glass beads | 20 kHz, 0–140 W | With glass beads, at saturation pressure 13.3 kPa, |
Nomura and Nakagawa [ | Cooling a narrow surface, acoustic streaming and cavitation effects separated, water tank, experimental investigations | 40 kHz, 600 W | Acoustic streaming at 0.4 m s−1, |
Nomura et al. [ | Downward facing surface, ultrasound from below, experimental, cavitation, and acoustic streaming | 60.7 kHz, 5–20 W | Up to 10-fold increase in heat transfer coefficient, tap and degassed water |
Nomura et al. [ | Turbulence intensity measured experimentally, square channel, transducer at the bottom | 25 kHz, 0–50 W | Turbulence intensity 3 times larger with ultrasonic vibrations and up to 5 times locally |
Nomura et al. [ | Effect of ultrasonic frequency on downward facing and vertical surface | 28 kHz (110 W), | Around 2 or 3 times average increase in |
Nomura et al. [ | Experimental, natural convection, obstacle in front of a heating surface (different materials), acoustic streaming | 60.7 kHz, 5–20 W | Up to 3 times with acrylic plate at 20 W, obstruction plates placed near the horn tip |
Richardson [ | Horizontal heated cylinder, horizontal and vertical sound fields, shadowgraphs | 710 and 1470 Hz | Local changes of boundary layer thickness and heat transfer enhancement |
Uhlenwinkel et al. [ | Experimental, gas vessel (air argon helium), resonant acoustic field, distance between transducers 20–200 mm | 10 and 20 kHz | Heat transfer enhancement up to 25 times at ambient pressure at about 0.9 MPa and 20 kHz |
Vainshtein et al. [ | Two horizontal plates at different temperatures, acoustic streaming in longitudinal direction | 200 Hz–15 kHz, 140 and 145 dB | Nu from 1 to 10, increase with frequency |
Yukawa et al. [ | Inclined copper plate in water | 28 kHz, 0.1–0.48 W cm−2 | Convection coefficient increased 6-fold at inclination 90°, intensity 0.48 W cm−2 |
Zhou et al. [ | Horizontal copper tube in water, acetone and ethanol, experimental study | ? | Maximum ratio of heat transfer enhancement: 3.95 with acetone, maximum source intensity, and close sound distance |
Numerical simulation is taking a more and more important place with the growing potential of computational calculation. Even if the systems of interest often remain quite simple (one fluid, one moving part), the level of accuracy of computations can be very high [
A numerical model of acoustic streaming between two parallel beams separated by an air gap between 0.1 and 2 mm wide is proposed in [
The field synergy principle is also a convincing way to illustrate cavitation-enhanced heat transfer [
Summary of numerical researches on convection increase by ultrasound.
Reference | Description of the study | Frequency, power, intensity | Best and/or interesting result obtained |
---|---|---|---|
Aktas et al | Shallow enclosure, vibrating vertical side wall, acoustic streaming | 20 kHz and 25 kHz | After 5 ms, |
Cai et al. [ | Square enclosure—hot bottom, natural convection, acoustic cavitation, ultrasonic beam from the centre | 18 kHz | Field synergy principle analysis, 25% increase in |
Lin and Farouk [ | Gas-filled square enclosure vibrating side-wall, top-side heated | 20 kHz | Heat transfer enhanced with streaming flow velocity (maximum at the middle of the bottom wall) |
Wan and Kuznetsov [ | Acoustic streaming in a gap (0.1–2 mm) between two horizontal beams, the lower vibrating | 160 Hz | 1% increase in Nusselt number for constant heat flux case of the upper beam |
Wan and Kuznetsov [ | Air channel composed of two parallel beams, upper beam vibrating | 21 kHz | h from 0.9 to 82 W m−2 K−1 at constant heat flux, decreasing with channel width |
In the previous sections, examples concerned configurations with only one fluid in thermal contact with another solid body at a different temperature. It was necessary to gain a good knowledge of those basic systems before studying more complex ones. Heat exchangers have at least two fluids (flowing or at rest), which makes systems sometimes more tricky to study. Indeed, they are subjected to several constraints, and ultrasonic vibrations have influence on various parameters (e.g., heat transfer, fouling, and charge losses). It is, therefore, more difficult to assess the efficiency of ultrasound on such systems. That is probably one of the main reasons why their development is quite recent. This is the field of research that is currently under development in our laboratory.
One of the first studies was carried out by Kurbanov and Melkumov in 2003 [
Cooling of sonochemical reactors by cold water flowing into a coil, as presented in Figure
Ultrasonically assisted cooling of a chemical reactor.
A shell-and-tube configuration for a fluid-to-fluid vibrating heat exchanger was built and studied [
Schematic diagram of the vibrating shell and tube heat exchanger.
The ratio between the overall heat transfer coefficient with ultrasound and the one without ultrasound for this shell-and-tube heat exchanger was calculated and found ranging from 1.2 up to 2.6 depending on the liquid flowrate at the shell side [
As shown in [
Another important phenomenon resulting from ultrasonic vibrations application and not described until here is surface cleaning (essentially thanks to acoustic cavitation). This is very important because it could be part of a solution to reduce the natural fouling process in heat exchangers. Indeed, the environmental conditions in such devices make them prone to corrosion or microorganisms deposition. They induce additional thermal resistances which prevent the system from operating in optimal conditions, adding environmental and economical costs. However, one must pay attention to the powerful erosion capability of cavitation that could damage materials. Benzinger et al. [
Influence of ultrasound on pressure drop, or charge losses, also seems to be positive although very few studies deal with this subject [
Table
Review of vibrating structures for heat exchangers and their advantages.
Reference | Description of the study | Frequency, power, intensity | Best and/or interesting result obtained |
---|---|---|---|
Benzinger et al. [ | Microstructured heat exchanger, antifouling investigations | 20 kHz, 35 W | Pulses of 1 min to break the fouling layer but fouling speed increased |
Bott and Tianqing [ | Ozone + ultrasound to clean heat exchangers, axially propagated ultrasound | 20 kHz, 2357.8 kW m−2 | 2357.8 kW m−2, 3 × 1 min pulse/day, up to 70% reduction in biofilm thickness |
Bott [ | Control of biofilm formation or biofilm removing in heat exchangers | 20 kHz | 88% reduction of biofilm growth with 10 treatments/day, 3 × 30 s at 40% amplitude |
Gondrexon et al. [ | Vibrating shell-and-tube heat exchanger, experimental investigation | 35 kHz, 80 W | Overall heat transfer coefficient increased up to 257% |
Kurbanov and Melkumov [ | Heat exchanger-type for heating and refrigeration | 3 and 16 kHz | 27% increase in |
Li et al. [ | Effects of various parameters on antiscale and scale removal. | 14–20 kHz; | Larger acoustic intensity is better for scale removal. |
Monnot et al. [ | Cooling of chemical reactor (2.9 L), experimental and modelling | 800 kHz; 1.6 MHz; 20 kHz; 0–109 W | Max |
Mott et al. [ | Experimental investigation, glass tubes filled with water, standing waves | 20–350 kHz, | 95.3% of biofilm removed by |
Tisseau et al. [ | Shell and tube heat exchanger, experimental investigation | 35 kHz, variable power | Overall heat transfer coefficient increase up to 250% |
Assessments of all these advantages in academic research literature are rare. Nevertheless, several systems (setups) regarding vibrating heat exchangers have been recently patented [
Ultrasound has gained a growing interest from industry during the last decades, resulting in the development of several specific applications. Ultrasonic waves appear as an interesting way to improve processes productivity especially to overcome transfer limitations. For what concerns heat transfer, ultrasound can also be regarded as a possible technical solution for heat exchange enhancement. Hence, a lot of publications dealing with fundamental studies can be found in the literature. But most of these works are performed at the laboratory scale involving academic set-ups and usually using classical low frequency ultrasound. Well-known ultrasonicallyinduced effects such as acoustic cavitation, acoustic streaming, and fluid particles oscillations are responsible for heat transfer improvement observed. It is also very important to note here that it is very difficult to distinguish the influence of these effects since they often occur simultaneously. One might therefore consider the positive influence of ultrasound as an overall effect. As detailed in this paper, influence of ultrasound on convection remains the major subject of interest. Local heat transfer coefficient was shown to be multiplied between 2 and 5 times in the presence of an ultrasonic field. Phase change heat transfer also covers a great number of studies that demonstrate the beneficial effect of ultrasound on boiling as well as melting or solidification. A more recent and scarce research field that focuses on heat exchangers has shown that the use of ultrasonic waves can improve overall performances regarding heat transfer and/or fouling.
Although very promising results are reported, the scale-up of the ultrasonic technology to pilot or industrial scale heat exchangers has not been yet deeply investigated. Only few references are available in the literature, illustrating the difficulties to meet such a technological challenge. It is then expected that the combined efforts of acousticians, chemical and mechanical engineers will also help to design a new type of “vibrating” heat exchangers. It might, therefore, result in improved performances as well as antifouling action in the near future.
Convection heat transfer coefficient W m−2 K−1
Nusselt number
Power W
Temperature K or °C
Temperature difference K
Critical heat flux
Computational fluid dynamics
Ultrasound.
This work was supported by the association