Heat transfer mechanism driven by acoustic body force under acoustic fields

In this paper, we demonstrate a heat transfer mechanism using ultrasonic standing waves. The basic idea behind the proposed heat transfer mechanism is the acoustic relocation phenomenon of inhomogeneous fluid due to acoustic body force. The acoustic body force depends upon the density gradient and the speed of the sound gradient of the inhomogeneous fluid. Heating a fluid creates an inhomogeneity in the physical properties of the fluid such as density, viscosity, and velocity of sound, etc. When this heated (inhomogeneous) fluid is subjected to ultrasonic standing waves, acoustic body force induces a fluid motion which is shown to be responsible for this heat transfer mechanism. Heat transfer enhancement is observed when a standing acoustic wave is passed perpendicular to the direction of heat transfer. Remarkably, it is found that acoustic forces can enhance heat transfer up to 2.5 times compared to natural convection and up to 11.2 times compared to pure conduction. Suppression of natural convection heat transfer is observed when the acoustic waves are passed parallel to the direction of heat transfer. In this case, acoustic forces could bring down the heat transfer by half or more than half from the natural convection. To characterize the heat transfer mechanism in the enhancement case, a modified Rayleigh number that can account for both acoustics and gravity effects is proposed. To this extent, we provide a clear understanding of how acoustic fields influence the fluid flow and heat transfer.

Figure 1

Acoustic relocation phenomenon in the inhomogeneous fluids of different configurations subjected to acoustic standing half waves. (a) High-density fluid initially at sides (antinode). (b) High-density fluid initially at the center (node) and (c) high-density fluid initially at the bottom. Green dotted lines indicate standing half waves whereas the green arrow indicates the direction of standing half waves. The yellow long dot-dashed line indicates node (center) and the yellow short dashed line indicates antinode (sidewalls).

Figure 2

Schematic representation of the proposed heat transfer mechanism based on acoustic relocation phenomenon.

Figure 3

Schematic of the 2D minichannel of width 8 mm along $x$ direction and height of 4 mm in the $y$ direction. (a) Acoustic standing half wave applied along X axis or perpendicular to the direction of heat transfer. (b) Acoustic standing half-wave applied along Y axis or parallel to the direction of heat transfer.

Figure 4

Grid independence test for heat flux in ethanol. (a) Acoustic wave perpendicular to the direction of heat transfer. (b) Acoustic wave parallel to the direction of heat transfer. The red dot indicates the number of grids chosen, resulting in a fine computational mesh for above dependent variables to reach convergence.

Figure 5

Velocity profile due to acoustic forces at zero gravity condition. Standing acoustic wave is applied perpendicular to the direction of heat transfer. (a) Ethanol and (b) water. The green arrow indicates the direction of the standing acoustic wave.

Figure 6

Acoustic properties of ethanol and water at different temperatures. (a) Density of water and ethanol at different temperatures and (b) velocity of sound in water and ethanol at different temperatures.

Figure 7

Density, speed of sound, and temperature profile of ethanol and water at $50\mathrm{J}/{\text{m}}^3$ when the acoustic wave is applied perpendicular to the direction of heat transfer. (a) Ethanol and (b) water.

Figure 8

Velocity profile due to the presence of both acoustics and gravity when the standing acoustic wave is applied perpendicular to the direction of heat transfer (a) ethanol and (b) water. The green arrow indicates the direction of the standing acoustic wave.

Figure 9

Heat flux (spatiotemporal average) at different acoustic energy densities when the acoustic wave is applied perpendicular to the direction of heat transfer (a) ethanol and (b) water.

Figure 10

Variation of Nusselt number (spatiotemporal average) for different modified Rayleigh numbers when the acoustic standing wave is perpendicular to the direction of heat transfer.

Figure 11

Velocity profile due to acoustic forces at zero gravity condition. Standing acoustic wave is applied parallel to the direction of heat transfer. (a) Ethanol and (b) water. The green arrow indicates the direction of the standing acoustic wave.

Figure 12

Density, speed of sound, and temperature profile of ethanol and water at $50\mathrm{J}/{\text{m}}^3$ when the acoustic wave is applied parallel to the direction of heat transfer. (a) Ethanol and (b) water.

Figure 13

Velocity profile due to the presence of both acoustics and gravity. Standing acoustic wave is applied parallel to the direction of heat transfer. (a) Ethanol and (b) water. The green arrow indicates the direction of the standing acoustic wave.

Figure 14

Heat flux (spatiotemporal average) for different acoustic energy densities. Acoustic wave is applied parallel to the direction of heat transfer. (a) Ethanol and (b) water.