Jamal Seyed-Yagoobi

This study numerically explores the effect of presence of micro-encapsulated phase change material (MEPCM) on the heat transfer characteristics of a fluid in a rectangular cavity driven by natural convection. The natural convection is generated by the temperature difference between two vertical walls at constant temperatures. The phase change material (PCM) melts in the vicinity of the hot wall and solidifies near the cold wall. Unlike the pure fluids, the heat transfer characteristics of MEPCM slurry cannot be simply presented in terms of corresponding dimensionless controlling parameters such as Rayleigh number. In the presence of phase change particles, the controlling parameters’ values change significantly due to the local phase change. The numerical results show significant increase in the heat transfer coefficient (up to 80%) at the considered operating conditions. This increase is a result of the MEPCM latent heat and the increased volumetric thermal expansion coefficient due to MEPCM volume change during melting.

Electrohydrodynamic conduction phenomenon takes advantage of the electrical Coulomb force exerted on a dielectric liquid generated by externally applied electric field and dissociated charges from electrolytes. The electric conduction phenomenon can be applied to enhance or control mass transport and heat transfer in both terrestrial and microgravity environments with advantages of simplicity and no degradation of fluid properties for isothermal as well as nonisothermal liquids. This paper numerically studies the heat transfer augmentation of externally driven macro- and microscale parallel flows by means of electric conduction phenomenon. The electric conduction is generated via electrode pairs embedded against the channel wall to mainly enhance the heat transfer and not necessarily to pump the liquid. Two cases of Poiseuille and Couette parallel flows are considered where for the former, a constant external pressure gradient is applied along the channel and for the latter, the channel wall moves with a constant velocity. The electric field and electric body force distributions along with the resultant velocity fields are presented. The heat transfer enhancements are illustrated under various operating conditions for both macro- and microscales.

Effect of electric conduction phenomenon on the mixing mechanism is studied numerically to thermally homogenize a dielectric liquid with an initial nonuniform temperature distribution. The fluid is stored in a spherical reservoir, and the electrodes are embedded on the reservoir surface such that the resultant local electric body forces mix the fluid. The electric field and electric body force distributions along with the resultant velocity field at the final steady-state condition are presented. The mixing mechanism is illustrated by the time evolution of temperature distribution inside the reservoir. The effects of primary dimensionless numbers on the mixing time are studied.

Electrohydrodynamic (EHD) conduction phenomenon takes advantage of the electrical Coulomb force exerted on a dielectric liquid generated by externally applied electric field. The conduction phenomenon can be applied to enhance or control mass transport and heat transfer in both terrestrial and microgravity environments with advantages of simplicity and no degradation of fluid properties for isothermal as well as non-isothermal liquids. This paper numerically studies the heat transfer augmentation of externally driven macro- and micro-scale parallel flows by means of electric conduction phenomenon. The electric conduction is generated via electrode pairs embedded against the channel wall to solely enhance the heat transfer; it is not utilized to pump the liquid. Two cases of Poiseuille and Couette parallel flows are considered where for the former, a constant external pressure gradient is applied along the channel and for the latter, the channel wall moves with a constant velocity. The electric field and electric body force distributions along with the resultant velocity fields are presented. The heat transfer enhancements are illustrated under various operating conditions for both scales.

Electrohydrodynamic (EHD) conduction pumping is associated with the heterocharge layers of finite thickness in the vicinity of the electrodes, generated by the process of dissociation of the neutral electrolytic species and the recombination of the generated ions. This paper numerically investigates the EHD-conduction pumping of a liquid film in the presence of evaporation. The flow system comprises a liquid film flowing over a two-dimensional flat plate. The vapor phase above the flat plate is extended far beyond the interface. The channel is separated into four different sections: the entrance, electrode, evaporation, and downstream sections. The entrance, electrode, and downstream regions are adiabatic while a constant heat flux is applied in the evaporation section. The concept of EHD-conduction pumping of liquid film in the presence of phase change is numerically demonstrated in this paper. The resultant heat transfer due to conduction pumping is evaluated as well. The results for heat transfer coefficient along the channel indicate considerable improvement of heat transfer coefficient compared with the pressure-driven counterpart.

A three-dimensional mathematical model is presented that models bubble deformation of a dielectric fluid due to the presence of a nonuniform electric field and calculates the net dielectrophoretic force that is exerted by the electric field on the bubble. The study includes the development of a method of predicting the shape of a bubble based on the arbitrary distribution of stresses over its surface without requiring an axisymmetric configuration. The reciprocal effect of the bubble’s presence on the electric field is also incorporated into the model, and dimensional analysis is used to obtain a single key parameter that governs the bubble deformation phenomenon. Numerical implementation of the mathematical model shows that the bubble deformation can be significant. Furthermore, bubble deformation and electric field distortion can have significant effects on the dielectrophoretic behavior of bubbles in nonuniform fields, especially within small-scale devices where the bubble size and electrode spacing are similar in magnitude.

Electrohydrodynamic (EHD) conduction phenomenon involves the interaction of electric field and flow field in a dielectric fluid medium via the process of dissociation and recombination of free charges. This paper numerically studies the effect of electric conduction phenomenon on the mixing mechanism of two fluids with identical physical properties but separated due to the non-homogeneity of the temperature field. The fluid is designated to be restored in a spherical reservoir and it is not spontaneously mixed since the reservoir is predicted to be located in non-gravity environment. The electrodes are embedded on the reservoir surface such that the resultant electric body force causes the fluid with higher temperature mixes with the colder fluid and vice versa. The electric field and electric body force distribution and the resultant velocity field are presented. The results are illustrated in the form of time evolution of temperature distribution inside the reservoir. The effects of primary dimensionless numbers on the mixing time are studied.

Heat pipes are well-known as simple and effective heat transport devices, utilizing two-phase flow and the capillary phenomenon to remove heat. However, the generation of capillary pressure requires a wicking structure and the overall heat transport capacity of the heat pipe is generally limited by the amount of capillary pressure generation that the wicking structure can achieve. Therefore, to increase the heat transport capacity, the capillary phenomenon must be either augmented or replaced by some other pumping technique. Electro-hydrodynamic (EHD) conduction pumping has been demonstrated as an effective method for pumping liquid films by using DC electric fields and a dielectric working fluid. Beyond increased pumping capacity, EHD conduction pumping offers other advantages over capillary pumping, such as active control of the pumping capacity via the intensity of the applied electric field. This experimental study demonstrates the prospects of a macro-scale two-phase heat transport device that is driven by EHD conduction pumping. Various liquid film thicknesses are considered. In each case, the performance of the EHD-driven heat transport device at various electric field intensities is compared to the capabilities of the same device under gravity alone. The effect of tilt on the device is also considered.

Electrohydrodynamic (EHD) conduction pumping is associated with the heterocharge layers of finite thickness in the vicinity of the electrodes, generated by the process of dissociation of the neutral electrolytic species and recombination of the generated ions. This paper numerically investigates the effect of EHD conduction pumping of liquid condensate circumferentially on the heat transfer performance of a condensation tube. The local interface profile is determined by the balance of fluid velocity and inter-phase momentum exchange. The conduction mechanism is designated to pump the liquid film from the top portion of the tube where the gravity body force plays minimal role. The interface profile, liquid condensate flow structure, and heat transfer rate under operating of one electrode pair and two electrodes pairs are investigated. The heat transfer performance for various operating conditions is also studied in this paper.

It is widely known that dielectrophoretic force can be harnessed to enhance the separation of liquid and vapor phases, with several known benefits in heat transfer enhancement. It has been shown that, when the electrode spacing and the bubble radius are of the same order of magnitude, the presence of the electric field can significantly deform the bubble, and this deformation can significantly affect the bubble’s behavior. Additionally, the presence of a bubble can provide significant, local distortion of the electric field. Consequently, the existence of multiple bubbles in close proximity may generate interactions that serve to further affect bubble deformation and motion behavior. Of course, nucleate boiling involves the generation of several bubbles in close proximity, and it is useful to understand whether these interactions may favorably or adversely affect the potential for heat transfer enhancement. This numerical study simulates the behavior of two and three bubbles within an external, electric field. The geometric deformation of the bubbles due to the electric field and the distortion of the electric field due to the existence of the bubbles are both incorporated into the mathematical model. The numerical results provide information about the effect that one bubble can have on the others’ motion, and also illustrate any tendencies of the bubbles to attract or repel each other when subject to various electric fields. Based on the numerical results, conclusions are drawn on the implications that the observed phenomenon may have on heat transfer enhancement applications.