This thesis makes a novel contribution to the state-of-the-art literature on EHD melting enhancement of PCMs showing the effects of electroconvection flow and solid extraction during the melting process. The details of the contribution made by this work have been disseminated in the form of three journal publications, which have been integrated into this sandwich Ph.D. thesis. / Latent heat thermal energy storage plays an important role in bridging the gap between the energy supply and consumer demands. The latent heat storage systems use phase change materials (PCMs) which are characterized by their high latent heat and therefore lead to higher energy densities. However, one major disadvantage of PCMs is their low thermal conductivities which affects the rates of charging and discharging. Electrohydrodynamics (EHD) offers an opportunity as an active heat transfer enhancement method which can significantly enhance the melting rates while being able to control the heat transfer as per the system’ needs with a very low power consumption. The application of EHD in two-phase solid liquid systems results in generating electroconvection flow in the liquid medium which increases the heat transfer coefficient and decreases the melting time.
The main objective of the current work is to study the heat transfer enhancement and the role of EHD forces during the melting of phase change materials (PCMs) under constant temperature boundary conditions. There are two main investigations performed in the current study. First is experimentally studying the EHD melting enhancement of PCMs while applying high voltages through two rows of electrodes embedded inside the PCM. Moreover, in the experiments, solid extraction was investigated using high-speed imaging conducted at various locations with respect to the electrodes. In the second investigation, PCM melting in a rectangular cavity under the effect of EHD and constant temperature boundary conditions is studied numerically. The flow field, temperature field, and phase field are simulated during the melting process until a steady state condition is reached. Additionally, the effect of the applied voltage and temperature boundaries on the electroconvection flow is illustrated.
Experimentally, the EHD melting enhancement of paraffin wax is examined under different applied DC voltage magnitudes and polarities, and different temperature gradients. In addition, the role of EHD forces was investigated by applying DC and AC square waves with different frequencies and offset values. The results showed that the melting enhancement increases with a nonlinear relation with voltages, wherein the maximum effective thermal conductivity was found to be 0.95 W/m-K at -10 kV in comparison with the value of 0.2 W/m-K for the pure liquid paraffin wax, with an enhancement ratio of 4.75. The Coulomb force was concluded to be the dominant EHD force in the study while the dielectrophoretic effect was negligible.
Characterization of solid extraction was performed by measuring the intensity of extraction, and the size and velocity of dendrites after extraction at different applied voltages and temperature boundaries for different phase change materials having different mushy zone thickness. For paraffin wax, solid extraction was detected for all the applied DC voltages. Small dendrites were observed to be pulled out from the mushy zone melt front and rise upwards in a rotational manner. The extraction intensity was found to be high at locations of high Coulomb force near the electrodes. In addition, solid extraction measurements showed that the size and velocity of the extracted dendrites increase alongside the applied voltage while the velocity decreases at higher temperature boundaries. Finally, it was found that the existence of a large mushy zone results in higher solid extraction intensities.
A numerical model was conducted using the finite element method to investigate the EHD melting of PCMs. In the model, the non-autonomous charge injection assumption is used with the Coulomb force being the only electrical body force considered. First, phase-change modeling is conducted to simulate the melting of paraffin wax without EHD under constant temperature boundary conditions until a steady-state condition is achieved. Next, the whole set of coupled EHD equations is introduced to the model to simulate the EHD melting process. The results revealed that two electroconvection cells were generated between each two successive electrodes in the liquid PCM. The EHD flow leads to the redistribution of the temperature field which enhances the heat transfer. EHD melting continues until a steady-state condition is regained after one hour of EHD time, at which point the enhancement ratio was found to be 2.33 at 6 kV. The influence of the applied voltages and temperature boundaries on the electroconvection flow showed that the fluid velocity increases significantly by increasing the voltage while it decreases under higher temperature gradients across the liquid region.
This thesis makes a novel contribution to the state-of-the-art literature on EHD melting enhancement of PCMs showing the effects of electroconvection flow and solid extraction during the melting process. The details of the contribution made by this work have been disseminated in the form of three journal publications, which have been integrated into this sandwich Ph.D. thesis. / Thesis / Doctor of Philosophy (PhD)
| Identifer | oai:union.ndltd.org:mcmaster.ca/oai:macsphere.mcmaster.ca:11375/29374 |
| Date | January 2024 |
| Creators | Hassan, Ahmed |
| Contributors | Cotton, James, Mechanical Engineering |
| Source Sets | McMaster University |
| Language | en_US |
| Detected Language | English |
| Type | Thesis |
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