EHD Effects Research Articles

Characterization of Wicking Performance for Open Rectangular Microgrooves under Planar EHD Effects in Two-Phase Heat Transfer Devices

Characterization of Wicking Performance for Open Rectangular Microgrooves under Planar EHD Effects in Two-Phase Heat Transfer Devices

Effects of the charge carrier elimination process on oil conductivity decrease under high electric field intensity

The DC conductivity of insulating oil is key to converter transformers, in which AC and DC electrical fields simultaneously exist. Research on the influence of the electric field on oil conductivity can optimise the insulation design of converter transformers. Although numerous studies have been conducted on the conductance mechanism of non-polar liquids, the explicit mechanism and properties of oil conductivity under a high electrical field remain unclear. This study developed an optimised conductance model considering charge carrier elimination, dissociation, and charge injection process. Thereafter, the authors chose three types of oils for converter transformers and measured the variations in oil conductivity and ion mobility under different electric fields with 0–10 kV/mm range. The result indicates that ion mobility will increase under a high electric field owing to the EHD effects. The conductivity of the insulating oil follows a U-shaped pattern with an increasing electric field, thereby generally matching the optimised theoretical model. Therefore, the decrease in conductivity is mainly the result of the elimination process of free ions, whereas the increase is the result of the injection process that occurs on the electrodes.

Electrohydrodynamic effects on a nanofluid-filled flat heat pipe

This study deals with a theoretical investigation on an axially grooved Flat Miniature Heat Pipe (FMHP) under an electric field. Copper oxide (CuO) and aluminum oxide (Al2O3) nanoparticles in N-pentane based nanofluid are considered. Under zero-electric field conditions, augmenting the nanoparticle volume fraction increases the liquid velocity at the evaporator and a part of the adiabatic zone, and reduces the vapor velocity in the adiabatic and the condenser zones. Results also reveal that the addition of the nanoparticles reduces the vapor pressure drop; in contrast, it increases the liquid pressure drop. The electric field increases the vapor pressure drop; however, it reduces the liquid pressure drop. Moreover, the EHD effects on the liquid and vapor velocities depend on the FMHP zone. The results also indicate that the capillary limit increases with the volume fraction of the nanoparticles and the electric field strength. It is also demonstrated that the capillary limit obtained with CuO nanoparticles is higher than that provided by Al2O3 nanoparticles. The study evidences the existence of an optimum fill charge for which maximum transferred heat powers are obtained. The optimum fill charge for the Al2O3 nanoparticles is lower than that obtained for CuO nanoparticles.

CVFEM approach for EHD flow of nanofluid through porous medium within a wavy chamber under the impacts of radiation and moving walls

In current investigation, ferrofluid circulation and energy transport inside a wavy-walled porous enclosure are modeled considering radiation and EHD effects. The finite volume method is employing for simulation of EHD circulation structures and thermal transmission. Properties of working fluid depend on the electric field and nanosized solid particles concentration. Impacts of thermal radiation, nanoparticles shape, and volume fraction are considered in governing equations. Distributions of unknown functions are received for various voltage, permeability, radiation parameters, nanoparticles’ shape and concentration. Results have shown that platelet form leads to the strongest convective circulation. An amplification of electric force characterizes a diminution of the boundary layer thickness. Greater permeability of the porous medium characterizes the strongest convective circulation and thermal transmission.

An experimental investigation on EHD effects in the thin-film region of an evaporating meniscus

The laser interferometric method is used to study EHD effects in the thin film region of an evaporating meniscus of DI water, which is formed in the triple-phase contact line region on a vertical heated wall. A columnar electrode, placed horizontally in air (at a relative humidity of 67%) and above the meniscus, is used as the high potential electrode, and the vertical surface coated with an ITO film is used as a heater as well as the low potential electrode. A DC voltage in the range of 0–4500V is applied to the electrode, and a heat flux of 1.4W/m2 and 5.6W/m2 respectively is applied to the ITO film. A high-speed camera is used to record the interference fringes in the thin-film region of the meniscus. It is found that effects of an imposed electric field are the decrease of the contact angle and the meniscus thickness while enhancing its evaporation rate and evaporative instabilities.

Analysis of a circular journal bearing lubricated with micropolar fluids including EHD effects

Purpose – The performance of finite circular journal bearing lubricated with micropolar fluids taking into account the elastic deformation of the bearing liner is presented. The paper aims to discuss these issues. Design/methodology/approach – The modified Reynolds equation is obtained using the micropolar lubrication theory. The solution of the modified Reynolds equation is determined using finite difference technique. The static characteristics in terms of load-carrying capacity, attitude angle, side leakage and friction coefficient for micropolar and Newtonian fluids are determined for various values of eccentricity ratio and different values of elastic coefficient. Findings – Compared with Newtonian fluids, the micropolar fluids produce an increase in the load-carrying capacity and a reduction in the attitude angle, the friction factor and side leakage for both the rigid and deformable bearings. Originality/value – It is concluded that the influence of elastic deformation on the bearing characteristics lubricated with micropolar fluids is significantly apparent compared with bearing lubricated with Newtonian fluids.

An experimental investigation on effects of an electric field on bubble growth on a small heater in pool boiling

EHD effects on bubble nucleation and its subsequent growth dynamic on a small heater in pool boiling of R113 are investigated experimentally in this paper. The small heater, 5mm×5mm in size, is made of an ITO film consisting of Pyrex glass and ITO-coatings. A non-uniform electric field is generated by a vertical needle at high potential and the small heater at low potential as electrodes. A high-speed camera is used to record the changing shape of a single R113 vapor bubble during its nucleation and its subsequent rise from the small heater at different heat fluxes with and without an externally imposed electric field. Effects of electric field strength and imposed heat flux on the bubble growth time, waiting time, departure frequency, dynamic contact angle and movement of triple contact line are presented. It is found that the electric field enhances nucleate boiling heat transfer of a single R113 bubble in the present experiment.

Electric Field Effects on Pool Boiling

This paper deals with an experimental study of the influence of a DC uniform electric field on the nucleate boiling heat transfer. EHD effects are quantitatively investigated by performing experiments on various liquids with different properties. In these experiments, n -pentane, R-113, and R-123 are used as working fluids, and the boiling phenomenon takes place on a horizontally oriented copper surface. The experimental results show (1) an enhancement of the nucleate pool boiling heat transfer, (2) an increase of the critical heat flux (CHF), and (3) disappearance of the hysteresis phenomenon. The basic enhancement mechanisms are analyzed and quantified. For nucleate pool boiling and CHF regimes, heat transfer laws based on dimensionless numbers are proposed and determined from the experimental data. The results obtained with the proposed EHD models are in good agreement with experimental results.

Experimental study on nucleate boiling enhancement and bubble dynamic behavior in saturated pool boiling using a nonuniform dc electric field

In order to investigate the effects of an electric field on the nucleate boiling heat transfer enhancement including bubble dynamic behavior, basic experiments have been performed under saturated pool boiling. In the present study, the working fluid is Freon-113 and the plate-wire electrode is used to generate a steep electric field gradient around the wire. A single-photo/high photography method is applied to measure boiling parameters. The experimental results show the shift of the boiling curve and the delay of ONB and CHF to a higher heat flux by an electric field. These EHD effects are much more considerable when the applied voltage increases. It is confirmed that the mechanisms of EHD nucleate boiling are closely connected with the dynamic behaviors of bubbles. Boiling parameters are investigated by using a high speed camera. The present results show that the boiling parameters are significantly changed by the electric field strength and nonuniformity. The relative contribution of latent heat to boiling heat transfer in an electric field is introduced. At very high voltage, the amount of latent heat transported by bubbles nearly corresponds to the total heat flux.

Electroconvective Instabilities in Freely Suspended and Bound Nematic Layers

EHD effects are studied in high conductivity MBBA layers aligned homeotropically with respect to all the interfaces. Both freely suspended and bound films are examined under ac fields acting transversely to the line of sight. The orientational distortion originates as two periodic waves close to the electrodes. The pattern symmetry is determined by their relative phase. For freely suspended films, both inphase and 180° out of phase waves occur; the domains are non-centrosymmetric in the former case, and centrosymmetric in the latter. For bound films, only the in phase waves occur; further, close to the frequency cut-off, two oppositely drifting wave trains appear along the electrodes.

Experimental study on nucleate boiling heat transfer enhancement using an electric field

To understand EHD nucleate boiling heat transfer enhancement, EHD effects on R-113 nucleate boiling heat transfer in a non-uniform electric field were investigated. The pool boiling heat transfer and the dynamic behavior of bubbles in d.c./a.c. electric fields under a saturated or subcooled boiling were studied by using a plate-wire electrode and a high speed camera. From the pool boiling heat transfer study, the shift of the pool boiling curve, the increase of the heat transfer and the delay of ONB and CHF points to higher heat fluxes were observed. From the dynamic behavior of bubbles, it was observed that bubbles departed away from the whole surface of the heated wire in radial direction due to EHD effects by a nonuniform electric field. With increasing applied voltages, the bubble size decreased and the active nucleation site and the departure number of bubbles showed the different trend. The present study indicates that the EHD nucleate boiling heat transfer is closely connection with the dynamic behavior of bubbles and the secondary flow induced near the heated surface. Therefore, the basic studies on the bubble behavior such as bubble frequency, bubble diameter, bubble velocity and flow characteristics are necessary for complete understanding of the enhancement mechanism of the boiling heat transfer using an electric field.

A dynamic analysis of tapered roller bearing under fully flooded conditions part 1: theoretical formulation

A vector method is used to solve the quasi-dynamic equilibrium in a tapered roller bearing. Besides the complex external loads the quasi-dynamic analysis includes both centrifugal forces and roller gyroscopic moment. The primary solutions obtained in this way are used in succession to carry out a complete dynamic analysis based on the integration process of the differential equations written for both rollers and cage. The numerical equalities proposed by Houpert ( Trans. ASME J. TriboL, 106 (1984) 375–385) are used to establish the lubrication regimes for both the roller-raceway contact and the rib-roller end contact. The sliding traction forces developed in the concentrated contacts are determined by assuming a nonlinear, viscoelastic lubricant. The shear stresses for both the unidirectional flow (line contact) and for the bidirectional flow (rib-roller end contact) are computed. The roughness effect of the contact surfaces, as well as the thermal EHD effects, are included. The results obtained in the dynamic analysis are presented in Part 2 of the paper and are compared with published data for analogous working conditions.

Stability study of an offset bearing including EHD effects

The present work reports the influence of deformation coefficient C̄d, a measure of flexibility of the bearing liner, on the threshold speed ( \\ ̄ gW) of a circular offset bearing. It is shown that the threshold speed improves with an increase in offset and/or the bearing flexibility. Further, the load-carrying capacity of a bearing is shown to depend largely on the deformation coefficient.

Advances in friction and wear mechanisms

This review considers what inferences regarding the mechanism of wear can be derived from recent investigations into the functioning of machine elements operating under conditions representative of engineering practice. Among the subjects discussed are particle analysis, surface texture effects, stress system, thermal effects, chemical aspects of wear, and ehd effects

Effect of duct configuration on oil activity at liquid/solid dielectric interfaces

A test cell has been designed to allow simultaneous determination of the currents flowing at the surfaces of two spacers in a 5mm oil gap. The spacers were asymmetrically disposed so their surfaces were at different distances from the bulk oil. Electrical measurements and Schlieren observations showed that the currents and activity in the oil at the spacer surface are influenced to a considerable degree by the length of duct: the current increasing by an order of magnitude in the longer duct. The technique tends to reduce the difficulties often experienced when attempting to compare results from direct voltage tests carried out in different cells. During the application of 50Hz alternating voltages to the system, different phenomena were observed at the spacers. These are considered to be due to the effect of dielectric losses in the spacer material. At higher stresses (of the order of 4 × 10 6V/m) EHD effects appeared to be present in certain regions resulting in movement of the oil: an increase in partial discharge activity was recorded for this condition. Higher frequency tests, up to 1000Hz, and similar stresses indicated that no EHD effects were present: it is postulated that the changes observed were due to losses in the oil itself. The work is continuing in an attempt to explain in more detail the various phenomena and to determine their possible significance in technology.

Effects of some chemical factors on film failure under EP conditions

This paper is primarily concerned with the effects of sulfur- or phosphorus-type additives and of the atmosphere on EP lubrication failure as an extension of work reported previously. With a modified stepwise loading procedure, the effects of some EP additives on wear and friction were investigated using a four-ball machine. It was found, from the results of electron probe microanalysis, that the chemical nature of films on rubbing surfaces varied with the atmosphere and with the chemical structure of the sulfur compounds. The nature of the surface films correlated closely with the tribological behavior and may control the lubrication regimes. The less reactive additives had better frictional behavior in air because EHD effects can contribute to the concentrated contact. The transitions observed here are compared with those proposed by OECD-IRG ∗ ∗ The International Research Group on Wear of Engineering Materials, under the auspices of OECD. . The method of evaluation of EP additives is discussed.

Critical temperatures in ep lubrication

This paper deals with EP lubrication failure as a temperature problem. In the Shell 4-ball machine using five sulfur compounds and two chlorine compounds two types of failure are recognized. Temperature calculations showed that the two failures occur at critical temperatures. To explain the constancy of the critical temperatures a different model for EP lubrication is proposed. Also under modified test conditions the possibility of EHD effects is recognized with some additives. An explanation for this effect is suggested. Electron probe micro analysis indicated the possibility of barrier films which may be responsible for the antiwear action of these additives leading to EHD effects.

The dependence on trace impurities of the observed carrier mobilities in n-hexane

The effects of impurities on mobilities measured in n-hexane using nano-second x ray ionization pulses are reported. A population redistribution occurred with the negative carrier species which was linearly dependent on impurity level. A connection between strong EHD effects and certain types of impurity is inferred.