Yildiz Bayazitoglu

Heat transfer in microtubes with viscous dissipation

Abstract Convective heat transfer for steady state, laminar, hydrodynamically developed flow in microtubes with uniform temperature and uniform heat flux boundary conditions are solved by the integral transform technique. Temperature jump condition at the wall and viscous heating within the medium are included. The solution method is verified for the cases where viscous heating is neglected. For uniform temperature case, with a given Brinkman number, at specified axial lengths, the viscous effects are presented for the developing range, reaching the fully developed Nusselt number. The effect of viscous heating is investigated for both of the cases where the fluid is being heated or cooled. Prandtl number analysis has shown that, as we increase the Prandtl number the temperature jump effect diminishes which gives a rise to the Nusselt number.

Forced Convection Cooling Across Rectangular Blocks

Conjugate heat transfer for two-dimensional, developing flow over an array of rectangular blocks, representing finite heat sources on parallel plates, is considered. Incompressible flow over multiple blocks is modeled using the fully elliptic form of the Navier-Stokes equations. A control-volume-based finite difference procedure with appropriate averaging for the diffusion coefficients is used to solve the coupling between the solid and fluid regions. The heat transfer characteristics resulting from recirculating zones around the blocks are presented. The analysis is extended to study the optimum spacing between heat sources for a fixed heat input and a desired maximum temperature at the heat source.

Heat transfer in rectangular microchannels

Abstract Convection heat transfer in a rectangular microchannel is investigated. The flow is assumed to be fully developed both thermally and hydrodynamically. The H2-type boundary condition, constant axial and peripheral heat flux, is applied at the walls of the channel. Since the velocity profile for a rectangular channel is not known under the slip flow conditions, the momentum equation is first solved for velocity. The resulting velocity profile is then substituted into the energy equation. The integral transform technique is applied twice, once for velocity and once for temperature. The results show a similar behavior to previous studies on circular microtubes. The values of the Nusselt number are given for varying aspect ratios.

A Review of Cooling in Microchannels

Advancements in electronic performance result in a decrease in device size and increase in power density. Because of these advancements, current cooling mechanisms for electronic devices are beginning to be ineffective. Within the localized hot spots, the materials of the components are reaching temperature values that can lead to improper functioning of the device. Many techniques have been successful in the past, such as heat sinks, cavities or grooves, micro pin-fins, etc., but still do not provide adequate cooling necessary to maintain temperature values low enough for the electronic components to operate. Microchannels, with their large heat transfer surface to volume ratio, cooled with either gas or liquid coolant, have shown some potential. By modifying the walls of the microchannel with fins, pins, or grooves, the cooling performance can be improved. A possible fin material used to increase the surface area of a microchannel is carbon nanotubes, which possess excellent thermal and mechanical properties. Numerical and computational methods needed to analyze flow at the micro- and nano-scale are also introduced. The numerical methods such as lattice Boltzmann, molecular dynamics, and computational fluid dynamics may lessen the cost and time that often accompany experimentation.

Effect of static deformation and external forces on the oscillations of levitated droplets

The oscillations of an aspherical droplet subjected to different external forces are considered. For an arbitrary shape deformation, it is shown that the frequency spectrum splits into (2l−1) peaks for a mode l oscillation, and the splitting of the frequency spectrum is calculated for mode 2, 3, and 4 oscillations. The deformation is then treated as a consequence of a general external force, and the frequency split is obtained in terms of the external force parameters. Droplets levitated by acoustic, electromagnetic, and combined acoustic‐electromagnetic forces are considered in particular, and it is shown that the effects of asphericity adequately explain the splitting of the frequency spectrum observed commonly in experiments. The interpretation of spectra with regard to accurate surface tension measurement using the oscillations of levitated droplets is discussed, and the results applied to some previous experimental results. It is shown that the accuracy of surface tension measurements can improve if ...

Numerical investigation of nanoparticle-assisted laser-induced interstitial thermotherapy toward tumor and cancer treatments

In this work, we numerically investigated nanoparticle-assisted laser-induced interstitial thermotherapy for tumor/cancer treatments. The goal of the study was to investigate the therapeutic effects of treatment conditions including laser wavelength, power, exposure time, concentrations of tailored nanoparticles, and optical/thermal properties of the tissue that is under treatment. It was found that using absorbing preferential nanoparticles as the photothermal agent weakens fluence rate distributions in terms of lowering fluence rate peaks and reducing laser penetration depths. However, the local enhancement in laser photon absorption induced by nanoparticles is so significant that the reduced fluence rate will be balanced out, and the eventual medical hyperthermia is greatly prompted by using nanoparticles. Also, the results of numerical simulations indicated that with constant laser illumination, an increase in nanoparticle concentration beyond a certain range has only an insignificant impact on hyperthermia.

Surface tension and viscosity from damped free oscillations of viscous droplets

Damped oscillations of a viscous droplet in vacuum or in an inert gas of negligible density are considered. The dependence of the complex decay factor on the properties of the liquid is investigated for the first time, and numerical results are compared with earlier studies for special cases. A new method is developed to determine both surface tension and viscosity from a single experiment in which the damping rate and frequency of oscillations are measured. The procedure to determine surface tension and viscosity from oscillating levitated liquids is outlined, and results are presented for various modes of shape oscillations.

Determination of surface tension from the shape oscillations of an electromagnetically levitated droplet

In the fundamental (l=2) mode, the frequency spectrum of a magnetically levitated inviscid droplet exhibits three distinct peaks. If the modes that correspond to each of these peaks is known, the surface tension of the droplet may be calculated. In experiments that make use of this principle, there is no unambiguous method of assigning mode numbers to these peaks. The dynamics of the oscillating droplet depend on the magnetic pressure on the droplet surface. Consequently, the order of the peaks in the l=2 mode oscillations is determined by the magnetic pressure distribution. In this paper, the magnetic pressure distribution on the surface of the droplet is calculated as a function of the parameters that govern the external magnetic field. The frequencies of the droplet oscillation and its static shape deformation are also expressed in terms of these same parameters. The frequencies of oscillation are used to determine the surface tension of the liquid droplet. Finally, the magnetic pressure distribution o...

Molecular dynamics simulation of an evaporating sodium droplet

Abstract Developments and advancements have recently been made on the nanoscale level, particularly in the area of the thermal sciences. Since continuum mechanics fail in such phenomena, a demand for molecular level analysis has been created. Molecular dynamics simulation has proven to be a viable means of microscopic analysis, due primarily to the advanced design of high speed computers. A molecular dynamics simulation is performed to analyse the limits of macroscopic behaviour of an isolated evaporating liquid sodium droplet. Lennard-Jones 12-6 potential is used to determine the intermolecular forces. Details of the simulation are presented as well as variations in properties obtained from the simulation. Parameters such as the bulk liquid density, vapour density, vapour pressure, surface tension, and interfacial surface thickness with respect to temperature are determined. Comparisons of the simulation results to the limiting macroscopic properties are made and trends in the data are discussed.

Effect of physiology on the temperature distribution of a layered head with external convection

Abstract To improve the existing thermal models of the human head, we incorporate the effect of the temperature over the metabolic heat generation, the regulatory processes that control the cerebral blood perfusion and their dependence on physiological parameters like, the mean arterial blood pressure, the partial pressure of oxygen, the partial pressure of carbon dioxide, and the cerebral metabolic rate of oxygen consumption. The introduction of these parameters in a thermal model gives information about how specific conditions, such as brain edema, hypoxia, hypercapnia, or hypotension, affect the temperature distribution within the brain. Our work, on a layered head model, shows that variations of the physiological parameters have profound effect on the temperature gradients within the head.