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.

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Enhancement of Heat Transfer by Ultrasound: Review and Recent Advances

A review of analytical, numerical and experimental investigations of melting and ensuing convection of phase change materials within enclosures with different shapes commonly used for thermal energy storage is presented. The common shapes of the containers being rectangular cavities, spherical capsules, tubes or cylinders (vertical and horizontal depending on orientation of gravity) and annular cavities are covered. Studies focusing on melting within rectangular cavities are categorized into two groups. The first one is melting due to isothermal heating on one or more boundaries, whereas the second is the constant heat flux-assisted melting. Moreover, constrained and unconstrained melting in both spherical and horizontal cylindrical containers were discussed. The effects of the concentric geometry and location of the heating source on melting in horizontal annular spaces are presented. The review concentrated on elucidating the heat transfer mechanisms (conduction and convection) during the multiple stages of the melting process and the effects of these mechanisms on the liquid–solid interface shape and its progress, melting rate, charging time of the storage system, etc. The strength of buoyancy driven-convection varies greatly with the dimensionless Rayleigh or Stefan numbers and depends somewhat on the location of heat source and imposed boundary condition. High dimensionless numbers and/or side position of the heat source ensure the dominant role of natural convection melting, otherwise conduction will be responsible for major melting within the container. Furthermore, the geometrical parameters such as the aspect ratio in rectangular containers and vertical cylindrical ones, diameter or radius in spherical capsules and horizontal cylindrical vessels, and eccentricity in annular cavities are reviewed. In addition, the parameters affecting the thermal behavior of the melting process in various enclosures, i.e. the Nusselt, Rayleigh, Stefan, Prandtl and Fourier numbers and are reviewed.

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Melting and convection of phase change materials in different shape containers: A review

Heat transfer enhancement of copper nanofluid with acoustic cavitation

A review on macro-encapsulated phase change material for building envelope applications

Experimental and numerical investigation of melting of phase change material/nanoparticle suspensions in a square container subjected to a constant heat flux

Effect of acoustic cavitation on boiling heat transfer

The melting process of n-octadecane in a rectangular cavity with three discrete protruding heat sources on its bottom surface was studied experimentally. It was observed that the experimental process, for the geometric arrangement in this paper, is neither a fixed melting nor a contact melting, but one in which fixed melting and contact melting take place alternatively. The effects of Stefan number, initial subcooling and aspect ratio on the melting process are reported. The larger the Stefan number, the more frequently the contact melting may occur, so does the aspect ratio. The initial subcooling plays a role only in early stage. As the melting process proceeds, its effect on the melting process becomes less.

Experimental study on melting in a rectangular enclosure heated below with discrete heat sources

In this paper, the authors have analyzed the second-order hyperbolic differential equation with pulsed heating boundary, and tried to solve this kind of equation with numerical method. After analyzing the error of the difference scheme and comparing the numerical results with the theoretical results, the authors present some examples to show how the thermal wave propagates in materials. By analyzing the calculation results, the conditions to observe the thermal wave phenomena in the laboratory are conferred. Finally the heat transfer in a complex combined structure is calculated with this method. The result is quite different from the calculated result from the parabolic heat conduction equation.

Difference Scheme for Hyperbolic Heat Conduction Equation with Pulsed Heating Boundary

An experiment has been carried out to investigate the local convective heat transfer from a horizontal circular copper tube in an acoustic cavitation field. The effects of acoustical parameters (including sound source intensity, the vibrator location and sound distance), fluid temperature and thermophysical properties of working fluid on heat transfer enhancement were studied, as well as the variation of heat transfer rate with sound source intensity at constant heat flux. Results show that convection heat transfer was remarkably enhanced due to disturbance and impingement by cavitation bubble. Among these cases tested, the maximum augmentation ratio of 3.95 was reached for acetone with a cluster of cavitation bubbles impinging perpendicularly on the tube surface.

Local convective heat transfer from a horizontal tube in an acoustic cavitation field

The invention provides a method of controlling the boiling heat transfer by the acoustic cavitation and titanium dioxide nano-particles, and relates to the heat exchange technology. The method includes the steps of adding the titanium dioxide nano-particles to a liquid in a testing case, and generating an acoustic cavitation field to the liquid so as to control the boiling heat transfer to intensify or weaken. The method of the invention has two elements, i.e., an adjustable-frequency ultrasonic transducer capable of generating cavitation bubble bunches and particles with the size in the nanometer level. A device for realizing the method of the invention comprises the testing case, a heat transfer test surface, electric heaters, the ultrasonic transducer, a supersonic generator, an observation window, a condenser, a thermocouple, a pressure gauge and the nano-particles. The method can carry out intensifying and weakening control for single-phase convection heat transfer of the heat transfer surface and the nucleate boiling heat transfer, with high heat transfer intensification rate and high cooling heat flow density; and the method of the invention is an active complex heat transfer intensification method.

Dengying Liu

Papers

Effect of acoustic cavitation on boiling heat transfer

Experimental study on melting in a rectangular enclosure heated below with discrete heat sources

Difference Scheme for Hyperbolic Heat Conduction Equation with Pulsed Heating Boundary

Local convective heat transfer from a horizontal tube in an acoustic cavitation field

Method and device for acoustic cavitation and titanium dioxide nano particle controlling boiling and heat conduction