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We experimentally investigate the evaporation kinetics of a sessile ferrofluid droplet placed on a soft substrate in the presence of a time-dependent magnetic field. We use both bright field visualization techniques and μ-PIV analysis to gain qualitative as well as quantitative insights into the internal hydrodynamics of the droplet. The results show that the droplet evaporation rate is augmented significantly in the presence of a time-dependent magnetic field, attributed primarily to the enhanced internal flow advection. We show that the motion of the magnetic nanoparticles dictates the overall life-time of the evaporating ferrofluid droplet. At lower frequencies of the magnetic field, the magnetic nanoparticles move towards the magnet and agglomerate into a chain-like cluster formation, oriented according to the magnetic field lines. On the other hand, at higher frequencies, the magnetic nanoparticles do not have sufficient time to travel the whole characteristic length (droplet diameter). Consequently, we observe the presence of a critical frequency at which the perturbation time scale balances the advective time scale of the flow inside the droplet. We show that on account for this balance between the time scales, the droplet experiences a minimum life-time. Finally, we demonstrate that the evaporation kinetics of a ferrofluid droplet in the presence of a time-dependent magnetic field can be described through three distinguishable stages viz., the decreasing contact angle and variable radius zone, the decreasing contact angle and decreasing radius zone and the late mixed zone. The inferences drawn from this study could have far-reaching implications in fields ranging from biomedical engineering to surface patterning.

Convective flow of single-phase ferrofluids under the influence of constant and alternating magnetic field has attracted attention as an effective strategy for enhanced heat transfer in mini/micro thermal systems. In the present study, an attempt has been made to gain deep insight of the heat transfer characteristics of single-phase ferrofluid flow in a heated stainless steel tube under the influence of constant and time-varying magnetic field. The governing parameters are mainly the magnetic flux density (B) and perturbation frequency (f) of the applied magnetic field. Three magnetic flux density value of B = 0 G, 700 G and 1080 G have been used for constant magnetic field. Constant value of B = 1080 G was used for alternating magnetic field while, frequencies of applied magnetic field has been varied from 0.1 Hz to 5 Hz. Flow Reynolds number was kept constant to Re = 66. Some 2-D numerical simulations have also been performed to qualitatively support the experimental data. The study is focused to delineate the mechanism of augmentation of heat transfer through the interaction of available force fields, i.e., interplay of magnetic force and inertia of the flow, and also the effect of various time scales on the flow and thermal behavior. Major inferences of the study are (a) on the application of external magnetic (constant and alternating), heat transfer augments (b) existence of a threshold frequency of external magnetic field for maximum augmentation as outcome of advective time-scale and magnetic perturbation time-scale. InfraRed Thermography (IRT) has been used to measure the wall temperature, while, some bright field visualizations have also been done to qualitatively support the explanations of experimental data.

We report the experimental investigations on the mixing of a ferrofluid droplet with a non-magnetic fluid in the presence of a time-dependent magnetic field on an open surface microfluidic platform. The bright field visualization technique, in combination with the micro-PIV analysis, is carried out to explore the internal hydrodynamics of the ferrofluid droplet. Also, using the Laser-induced fluorescence (micro-LIF) technique, we quantify the mass transfer occurring between the two droplets, which in effect, determines the underlying mixing performance under the modulation of the frequency of the applied magnetic field. We show that the magnetic nanoparticles exhibit complex spatio-temporal movement inside the ferrofluid droplet domain under the influence of a time-dependent magnetic field, which, in turn, promotes the mixing efficiency in the convective mixing regime. Our analysis establishes that the movement of magnetic nanoparticles in presence of the time-periodic field strengthens the interfacial instability, which acts like a sparking agent to initiate an augmented mixing in the present scenario. By performing numerical simulations, we also review the onset of interfacial instability, mainly stemming from the susceptibility mismatch between the magnetic and non-magnetic fluids. Inferences of the present analysis, which focuses on the simple, wireless, robust, and low-cost open surface micromixing mechanism, will provide a potential solution for rapid droplets mixing without requiring pH level or ion concentration dependency of the fluids., 34 pages, 14 figures

Pulsating heat pipes (PHP) are now well-known devices in which liquid/vapor slug flow oscillates in a capillary tube wound between hot and cold sources. In this context, this paper focuses on the motion of the liquid plug, trapped between vapor bubbles, moving in capillary tubes, to try to better understand the thermo-physical phenomena involved in such devices. This study is divided into three parts. In the first part, an experimental study presents the evolution of the vapor pressure during the evaporation process of a liquid thin film deposited from a liquid plug flowing in a heated capillary tube: it is found that the behavior of the generated and removed vapor can be very different, according to the thermophysical properties of the fluids. In the second part, a transient model allows to compare, in terms of pressure and duration, the motion of a constant-length liquid plug trapped between two bubbles subjected to a constant difference of vapor pressure: the results highlight that the performances of the four fluids are also very different. Finally, a third model that can be considered as an improvement of the second one, is also presented: here, the liquid slug is surrounded by two vapor bubbles, one subjected to evaporation, the pressure in both bubbles is now a result of the calculation. This model still allows comparing the behaviors of the fluid. Even if our models are quite far from a complete model of a real PHP, results do indicate towards the applicability of different fluids as suitable working fluids for PHPs, particularly in terms of the flow instabilities which they generate.

In this paper, a ferrofluid-based cooling technique is proposed for solar photovoltaic (PV) systems, where ferrofluid flow can be easily altered by the application of an external magnetic field leading to enhanced heat transfer from the hot surface of PV systems. The effect of both constant and alternating magnetic field on ferrofluid flow through a minichannel is explored numerically in the present work. A detailed parametric study is performed to investigate the effect of actuation frequencies of alternating magnetic field (0.5–20 Hz) and Reynolds numbers (Re = 24, 60, and 100) on heat transfer characteristics of ferrofluid. An overall enhancement of 17.41% is observed for heat transfer of ferrofluid in the presence of magnetic field compared to the base case of no magnetic field. For the case of alternating magnetic field, a critical actuation frequency is observed for each Reynolds number above which heat transfer is observed to decrease. The enhancement or decrease in heat transfer of ferrofluid is found to depend on several factors such as actuation frequency of alternating magnetic field, Reynolds numbers of ferrofluid flow, and formation/dispersion of stagnant layers of ferrofluid at the magnet location. Preliminary visualization of ferrofluid flow is also carried out to provide a qualitative insight to the nature of transportation of ferrofluid in the presence of an alternating magnetic field.

Disturbing a single-phase laminar internal convective flow with a particular pulsating flow frequency alters the thermal and hydrodynamic boundary layer, thus affecting the inter-particle momentum and energy exchange. Due to this externally imposed flow disturbance, augmentation in the heat transfer may be expected. Obviously, parameters like pulsating flow frequency vis-a-vis viscous time scales and the imposed pulsating amplitude will play an important role. Conclusions from reported literature on this and related problems are rather incoherent. Lack of experimental data, especially in micro-/mini internal convective flow situations, with imposed flow pulsations, motivates this study. Non-intrusive infra-red thermography has been utilized to scrutinize heat transfer augmentation during single-phase laminar pulsating flow in a square mini-channel of cross-section 3 mm × 3 mm, electrically heated from one side by a thin SS strip heater (70 μm, negligible thermal inertia); all the other three sides of the channel are insulated. The study is done at different pulsating flow frequencies of 0.05 Hz, 1.00 Hz and 3.00 Hz (Wo = 0.8, 3.4 and 5.9, respectively). These values are chosen because pulsatile velocity profiles exhibit different characteristics for Wo > 1 (annular effect, i.e., peak velocity near the channel walls) and Wo < 1 (conventional parabolic profile). Local streamwise heat transfer coefficient has been determined using the time averaged spatial IR thermograms of the heater surface and the local fluid temperature, linearly interpolated from measured value of inlet and outlet bulk mean mixing temperature. It is observed that for measured frequency range, the overall enhancement in the heat transfer is not attractive for laminar pulsating flow in comparison to steady flow with same time-averaged flow Reynolds number. The change is either marginal or highly limited, primarily occurring in the developing length of the channel. Thus, the results suggest that heat transfer enhancement due to periodic pulsating flow is questionable, and at best, rather limited.

The present paper reports actuation of sessile ferrofluid droplets over a hydrophobic substrate in the presence of a time-dependent magnetic field generated by an electromagnet. The internal hydrodynamics of the ferrofluid droplet in the presence of magnetic field are measured using both bright field visualization and micro-particle image velocimetry (μ-PIV) techniques. During ON cycle of the magnetic field, bright field visualizations show the migration of nanoparticles towards the contact line near the vicinity of the electromagnet resulting in aggregation of nanoparticles inside the droplet. Similarly, aggregated nanoparticles at the contact line from the ON cycle are observed to disperse from the cluster of nanoparticles during the OFF cycle of the magnetic field. Both migration and dispersion of nanoparticles result in bulk motion inside the ferrofluid droplet during the ON and OFF cycle of the magnetic field. Velocity measurements from μ-PIV technique successfully validate the qualitative measurements of flow field from bright field visualization technique. A critical frequency is observed for the applied magnetic field above which negligible dispersion of nanoparticles resulted inside the ferrofluid droplet during the OFF cycle of the magnetic field.

Understanding the performance of Pulsating Heat Pipes (PHPs) requires spatio-temporally coupled, flow and heat transfer information during the self-sustained thermally driven flow of oscillating Taylor bubbles. Detailed local hydrodynamic characteristics are needed to predict its thermal performance, which has remained elusive. Net heat transfer in PHP is contributed by (a) pulsating/oscillating flow (b) distribution of different liquid slugs and bubbles, and, (c) phase-change process; however, its contribution is minimal. In fact, the former two flow conditions are largely responsible for heat transfer in PHPs; such flow conditions can be generated without phase-change and can also be studied independently to observe their explicit effects on PHP heat transfer. With this motivation, systematic experimental investigation of heat transfer is performed during (a) isolated Taylor bubble flow (b) continuous Taylor bubble flow and (c) pulsating Taylor bubble flow, at various frequencies (1 Hz to 3 Hz, as applicable for PHPs) inside a heated square mini-channel of cross-section size 3 mm × 3 mm. This study clearly reveals important insights into the PHP operation. Oscillating Taylor bubbles create significant disturbances in their wake which leads to local augmentation of sensible heat transfer. The implications of bubble length, wake characteristics, oscillating frequency and bubble slip velocity on the heat transfer augmentation and, in turn on thermal performance of PHPs can be clearly delineated from this study. The study also brings out the nuances in the estimation of true bubble slip under time varying Taylor bubble flows.

In mini/micro confined internal flow systems, Taylor bubble train flow takes place within specific range of respective volume flow ratios, wherein the liquid slugs get separated by elongated Taylor bubbles, resulting in an intermittent flow situation. This unique flow characteristic requires understanding of transport phenomena on global, as well as on local spatio-temporal scales. In this context, an experimental design methodology and its validation are presented in this work, with an aim of measuring the local heat transfer coefficient by employing high-resolution InfraRed Thermography. The effect of conjugate heat transfer on the true estimate of local transport coefficients, and subsequent data reduction technique, is discerned. Local heat transfer coefficient for (i) hydrodynamically fully developed and thermally developing single-phase flow in three-side heated channel and, (ii) non-boiling, air–water Taylor bubble train flow is measured and compared in a mini-channel of square cross-section (5 mm × 5 mm; D h = 5 mm, Bo ≈ 3.4) machined on a stainless steel substrate (300 mm × 25 mm × 11 mm). The design of the setup ensures near uniform heat flux condition at the solid–fluid interface; the conjugate effects arising from the axial back conduction in the substrate are thus minimized. For benchmarking, the data from single-phase flow is also compared with three-dimensional computational simulations. Depending on the employed volume flow ratio, it is concluded that enhancement of nearly 1.2–2.0 times in time-averaged local streamwise Nusselt number can be obtained by Taylor bubble train flow, as compared to fully developed single-phase flow. This enhancement is attributed to the intermittent intrusion of Taylor bubbles in the liquid flow which drastically changes the local fluid temperature profiles. It is important to maintain proper boundary conditions during the experiment while estimating local heat transfer coefficient, especially in mini-micro systems.

Taylor bubble flow takes place when two immiscible fluids (liquid–liquid or gas–liquid) flow inside a tube of capillary dimensions within specific range of volume flow ratios. In the slug flows where gas and liquid are two different phases, liquid slugs are separated by elongated Taylor bubbles. This singular flow pattern is observed in many engineering mini-/micro-scale devices like pulsating heat pipes, gas–liquid–solid monolithic reactors, micro-two-phase heat exchangers, digital micro-fluidics, micro-scale mass transfer process, fuel cells, etc. The unique and complex flow characteristics require understanding on local, as well as global, spatio-temporal scales. In the present work, the axial streamwise profile of the fluid and wall temperature for air–water (i) isolated single Taylor bubble and, (ii) a train of Taylor bubbles, in a horizontal square channel of size 3.3 mm × 3.3 mm × 350 mm, heated from the bottom (heated length = 175 mm), with the other three sides kept insulated, are reported at different gas volume flow ratios. The primary aim is to study the enhancement of heat transfer due to the Taylor bubble train flow, in comparison with thermally developing single-phase flows. Intrusion of a bubble in the liquid flow drastically changes the local temperature profiles. The axial distribution of time-averaged local Nusselt number ( Nu ¯ z ) shows that Taylor bubble train regime increases the transport of heat up to 1.2–1.6 times more as compared with laminar single-phase liquid flow. In addition, for a given liquid flow Reynolds number, the heat transfer enhancement is a function of the geometrical parameters of the unit cell, i.e., the length of adjacent gas bubble and water plug.

Nanofluids, stabilized suspensions of nanoparticles typically