Electrohydrodynamic Number Research Articles

Effects of Buoyancy on Electrohydrodynamic-Enhanced Forced Convection in a Horizontal Channel

Buoyancy effects on heat transfer enhancement using an electrohydrodynamic (EHD) technique is numerically examined for laminar forced convection in a horizontal channel. Attention is also focused on the effect of added buoyancy on the flow stability. The flow Reynolds numbers and thermal buoyancy strength considered are in the range of 6 x 10 2 ≤ Re ≤ 1.8 x 10 3 and 10 4 ≤ Gr ≤ 10 6 , respectively. The electrical field is generated by positive corona from a wire electrode charged with dc high-voltage (10 ≤ V 0 ≤ 17.5 kV). In terms of an EHD number, this corresponds to 0.36 < N ehd ≤ 23.56. The results show that heat transfer enhancement increases with the applied voltage. For a given electric field, oscillation in the flow and temperature fields is observed for flows at small Reynolds numbers. In addition, the flow and temperature fields become more unstable with an increase in the thermal buoyancy strength. Because of the existence of secondary flows, there is an improvement in heat transfer. However, it is observed that thermal buoyancy has a negligible effect on the heat transfer enhancement for Gr < 10 6

Particle Dynamics in an Electrohydrodynamic Flow Field investigated with a two‐component laser‐doppler velocimeter

AbstractA laser‐Doppler instrument has been used to measure the migration velocity of NaCl particles in an electrohydrodynamic flow field of an electrical precipitator. The measured average migration velocity of 1.40‐μm particles (number distribution median with a geometric standard devitation of 1.46) is approximately five to six times higher than the calculated steady‐state velocity for a 1.40‐μm particle, provided there is a saturation charge of at least 90f%. Further, the particle velocities in the main flow direction are also influenced by the electrical operation conditions. Both observations demonstrate the important role of the state of the electrohydrodynamic flow field (superposition of moving gas ions and neutral gas molecules) on the particle transport, characterized by the dimensionless electrohydrodynamic number NEHD. A comparison between six different electrohydrodynamic states revealed that NEHD ≈︁ 1 is a critical value for the mutual interactions between the gas ions and the neutral gas phase. Whereas for NEHD values &gt; 1 the stochastic particle motion is chiefly determined by the nonsteady‐state character of the negative corona, for NEHD values &lt; 1 the particle velocity fluctuations are governed by the turbulence level of the neutral fluid. These finding might be helpful in adjusting the operating conditions in electrical precipitators for and optimized particle separation.