Sameer Khandekar

Closed loop pulsating heat pipes: Part A: parametric experimental investigations

Abstract Closed loop pulsating heat pipes (CLPHPs) are complex heat transfer devices having a strong thermo-hydrodynamic coupling governing the thermal performance. In this paper, a wide range of pulsating heat pipes is experimentally studied thereby providing vital information on the parameter dependency of their thermal performance. The influence characterization has been done for the variation of internal diameter, number of turns, working fluid and inclination angle (from vertical bottom heat mode to horizontal orientation mode) of the device. CLPHPs are made of copper tubes of internal diameters 2.0 and 1.0 mm, heated by constant temperature water bath and cooled by constant temperature water–ethylene glycol mixture (50% each by volume). The number of turns in the evaporator is varied from 5 to 23. The working fluids employed are water, ethanol and R-123. The results indicate a strong influence of gravity and number of turns on the performance. The thermophysical properties of working fluids affect the performance which also strongly depends on the boundary conditions of PHP operation. Part B of this paper, which deals with development of semi-empirical correlations to fit the data reported here coupled with some critical visualization results, will appear separately.

Understanding operational regimes of closed loop pulsating heat pipes: an experimental study

Abstract Increasing performance of electronic components is resulting in higher heat flux dissipation. Two-phase passive devices are proven solutions for modern microelectronics thermal management. In this context, heat pipe research is being continuously pursued evolving newer solutions to suit present requirements. Pulsating heat pipes (PHPs), a relatively new and emerging technology is one such field of investigation. The operating mechanism of PHP is not well understood and the present state of the art cannot predict required design parameters for a given task. The aim of research work presented in this paper is to better understand these mechanisms through experimental investigations. Experiments were conducted on a PHP made of copper capillary tube of 2-mm inner diameter. Three different working fluids viz. water, ethanol and R-123 were employed. The PHP was tested in vertical (bottom heat mode) and horizontal orientation. The results strongly demonstrate the effect of input heat flux and volumetric filling ratio of the working fluid on the thermal performance of the device. Important insight into the operational regimes of the device has been gained.

Closed loop pulsating heat pipes Part B: visualization and semi-empirical modeling

Abstract Pulsating heat pipes have received growing attention from experimental and theoretical researchers in recent times. Behind its constructional simplicity lie the intriguingly complex thermo-hydrodynamic operational characteristics. Part A of this paper has presented the thermal performance results of a fairly large matrix of closed loop pulsating heat pipes. This paper, which is an extension of the previous work, first presents some more visualization results to highlight the complexities involved in mathematical formulation of the modeling problem. The phenomenological trends recorded in the visualization set-up are fully inline with the previous quantitative data of Part A. Critical review of the existing modeling approaches to CLPHPs is presented in the wake of these results. A detailed discussion follows on the important issues involved in the mathematical modeling of these devices. Then, semi-empirical correlations based on non-dimensional numbers of interest for predicting the thermal performance of CLPHPs are presented. Although there are limitations of the models presented herein, modeling by non-dimensional numbers seems to be most promising as compared to other existing techniques.

An insight into thermo-hydrodynamic coupling in closed loop pulsating heat pipes

A Closed Loop Pulsating Heat Pipe (CLPHP) is a complex heat transfer device with a strong thermo-hydrodynamic coupling governing its thermal performance. To better understand its operational characteristics, a two-phase loop is constructed with a capillary tube (ID = 2.0 mm) having no internal wick structure. The loop is heated at one end and cooled at the other and partially made up of glass to assist visualization. The working fluid employed is ethanol. It is concluded from the study that a two-phase loop does represent the thermo-fluidic characteristics of a multi-turn CLPHP. Dynamic two-phase instabilities are present in a two-phase loop also; although the number of turns in a CLPHP increases the level of internal perturbations. The existence of an optimum number of turns for a given heat throughput requirement is explained. Also, it is shown that classical thermodynamics based on quasi-equilibrium theory seems not to be sufficient for complete system analysis. The performance (i.e., overall thermal resistance) is strongly dependent on the flow pattern existing inside the tubes. The role of gravity in the operation characteristics is clarified.

Nano-Cutting Fluid for Enhancement of Metal Cutting Performance

Nano-cutting fluids are the mixtures of conventional cutting fluid and nanoparticles. Addition of the nanoparticles can alter wettability, lubricating properties, and convective heat transfer coefficient (cooling properties) of nano-cutting fluids. In the present work, nano-cutting fluid is made by adding 1% Al2O3 nanoparticles to conventional cutting fluid. The wettability characteristic of this nano-cutting fluid on a carbide tool tip is measured using the macroscopic contact angle method. Comparative study of tool wear, cutting force, workpiece surface roughness, and chip thickness among dry machining, machining with conventional cutting fluid as well as nano-cutting fluid has been undertaken. This study clearly reveals that the cutting force, workpiece surface roughness, tool wear, and chip thickness are reduced by the using nano-cutting fluid compared to dry machining and machining with conventional cutting fluid.

Dropwise Condensation Underneath Chemically Textured Surfaces: Simulation and Experiments

Experimental observations of dropwise condensation of water vapor on a chemically textured surface of glass and its detailed computer simulation are presented. Experiments are focused on the pendant mode of dropwise condensation on the underside of horizontal and inclined glass substrates. Chemical texturing of glass is achieved by silanation using octyl-decyl-tri-chloro-silane (C 18 H 37 C 13 Si) in a chemical vapor deposition process. The mathematical model is built in such a way that it captures all the major physical processes taking place during condensation. These include growth due to direct condensation, droplet coalescence, sliding, fall-off, and renucleation of droplets. The effects arising from lyophobicity, namely the contact angle variation and its hysteresis, inclination of the substrate, and saturation temperature at which the condensation is carried out, have been incorporated. The importance of higher order effects neglected in the simulation is discussed. The results of model simulation are compared with the experimental data. After validation, a parametric study is carried out for cases not covered by the experimental regime, i.e., various fluids, substrate inclination angle, saturation temperature, and contact angle hysteresis. Major conclusions arrived at in the study are the following: The area of droplet coverage decreases with an increase in both static contact angle of the droplet and substrate inclination. As the substrate inclination increases, the time instant of commencement of sliding of the droplet is advanced. The critical angle of inclination required for the inception of droplet sliding varies inversely with the droplet volume. For a given static contact angle, the fall-off time of the droplet from the substrate is a linear function of the saturation temperature. For a given fluid, the drop size distribution is well represented by a power law. Average heat transfer coefficient is satisfactorily predicted by the developed model.

Dropwise Condensation Studies on Multiple Scales

Recent advances in nanotechnology, chemical/physical texturing and thin film coating technology generate definite possibilities for sustaining a dropwise mode of condensation for much longer durations than was previously possible. The availability of superior experimental techniques also leads to deeper understanding of the process parameters controlling the relevant transport phenomena, the distinguishing feature of which is the involvement of a hierarchy of length/time scales, proceeding from nuclei formation, to clusters, all the way to macroscopic droplet ensemble, drop coalescence, and subsequent dynamics. This paper is an attempt to connect and present a holistic framework of modeling and studying dropwise condensation at these multiple scales. After a review of the literature, discussions on the following problems are presented: (i) atomistic modeling of nucleation; (ii) droplet–substrate interaction; (iii) surface preparation; (iv) simulation of fluid motion inside sliding drops; (v) experimental determination of the local/ average heat transfer coefficient; and (vi) a macroscopic model of the complete dropwise condensation process underneath horizontal and inclined surfaces. The study indicates that hierarchal modeling is indeed the way forward to capture the complete process dynamics. The microscopic phenomena at the three-phase contact line, leading to the apparent droplet contact angle, influence the shear stress and heat transfer. The nucleation theory captures the quasi-steady-state behavior quite satisfactorily, although the early atomistic nucleation was not seen to have a profound bearing on the steady-state behavior. The latter is strongly governed by the coalescence dynamics. Visual observation of dropwise condensation provides important information for building hierarchical models.

MEASUREMENT OF HEAT TRANSFER DURING DROP-WISE CONDENSATION OF WATER ON POLYETHYLENE

Heat transfer coefficients associated with drop-wise condensation are quite large. Because the ensuing driving temperature difference is small, experimental determination of heat transfer coefficient is a challenge. The statistical nature of droplet distribution in the ensemble contributes to the intricacy of analysis and interpretation. Against this background, the spatial distribution of temperature during drop-wise condensation over a polyethylene substrate was measured using liquid crystal thermography (LCT) simultaneously with actual visualization of the condensation process by videography. Experiments were conducted in such a way that pendant drops form on the underside of the liquid crystal sheet. Temperature variation at the base of the droplets, as small as 0.4 mm, were satisfactorily resolved. The signature of the drop shape was visible in the LCT images. The drop size distribution on the substrate was simultaneously visualized. Static contact angles of water on polyethylene are measured and drop shapes were estimated via a mathematical model for comparison. Using a one-dimensional heat transfer approximation, heat flux profiles through individual droplets were obtained. The temperature profiles from LCT combined with drop sizes from direct visualization provide sufficient data for understanding the heat transfer mechanism during drop-wise condensation. Results show that the measured heat flux as a function of drop diameter matches published data for large drop sizes but fails for small drops where the thermal resistance of the LCT sheet is a limiting factor. To a first approximation, the present work shows that drop size can be correlated to the local heat flux. Hence, the average heat flux over a surface can be obtained entirely from the drop size distribution.

Optimum Nusselt Number for Simultaneously Developing Internal Flow Under Conjugate Conditions in a Square Microchannel

A numerical study has been carried out to understand and highlight the effects of axialwall conduction in a conjugate heat transfer situation involving simultaneously develop-ing laminar flow and heat transfer in a square microchannel with constant flux boundarycondition imposed on bottom of the substrate wall. All the remaining walls of the sub-strate exposed to the surroundings are kept adiabatic. Simulations have been carried outfor a wide range of substrate wall to fluid conductivity ratio (k

Remote-Access Real-Time Laboratory: Process Monitoring and Control through the Internet Protocol

The development of a remote-access real-time laboratory (RART-Lab) is described, and a case study is presented of its application to a real-time mechanical engineering experiment, namely a study of thermo-hydrodynamics of flow through mini-channels. (The study of such flows is vital for many applications, ranging from electronics thermal management to fuel cells.) The RART-Lab concept encompasses data acquisition during the experiment, storage, post-processing and online transmission of data to multiple users logged on to their respective web browsers. Control of the experimental process parameters (e.g. liquid mass flow rate and heater power level) from one (or more) remote stations over the web in real time is also incorporated. Online video images of the experimental facility, visualization modules and color-indexed temperature data can be transmitted by webcam. The system developed has a friendly graphical user interface. It also allows transmission of process parameter alarm signals via an e-mail client server or via an SMS text message to a mobile telephone. Simultaneously, conventional chatting has also been incorporated to add vibrancy to inter-user communication. In addition, three-dimensional computational fluid dynamics simulations have also been done simultaneously with the real-time laboratory to mimic the experiments. Such an integrated development greatly widens the possibilities of collaborative research, development, simulation and experimentation, to overcome the need for the physical proximity of the experimental hardware and experimenters. Such generic tools not only make academic interactions and real-time data sharing more fruitful but also greatly facilitate joint research and development activities between academia and the industrial community.