Roop L. Mahajan

Thermophysical properties of high porosity metal foams

Abstract In this paper, we present a comprehensive analytical and experimental investigation for the determination of the effective thermal conductivity (ke), permeability (K) and inertial coefficient (f) of high porosity metal foams. In the first part of the study, we provide an analysis for estimating the effective thermal conductivity (ke). Commercially available metal foams form a complex array of interconnected fibers with an irregular lump of metal at the intersection of two fibers. In our theoretical model, we represent this structure by a model consisting of a two-dimensional array of hexagonal cells where the fibers form the sides of the hexagons. The lump is taken into account by considering a circular blob of metal at the intersection. The analysis shows that ke depends strongly on the porosity and the ratio of the cross-sections of the fiber and the intersection. However, it has no systematic dependence on pore density. Experimental data with aluminum and reticulated vitreous carbon (RVC) foams, using air and water as fluid media are used to validate the analytical predictions. The second part of our paper involves the determination of the permeability (K) and inertial coefficient (f) of these high porosity metal foams. Fluid flow experiments were conducted on a number of metal foam samples covering a wide range of porosities and pore densities in our in-house wind tunnel. The results show that K increases with pore diameter and porosity of the medium. The inertial coefficient, f, on the other hand, depends only on porosity. An analytical model is proposed to predict f based on the theory of flow over bluff bodies, and is found to be in excellent agreement with the experimental data. A modified permeability model is also presented in terms of the porosity, pore diameter and tortuosity of our metal foam samples, and is shown to be in reasonable agreement with measured data.

Forced Convection in High Porosity Metal Foams

We report an experimental and numerical study of forced convection in high porosity (e∼0.89-0.97) metal foams. Experiments have been conducted with aluminum metal foams in a variety of porosities and pore densities using air as the fluid medium. Nusselt number data has been obtained as a function of the pore Reynolds number. In the numerical study, a semi-empirical volume-averaged form of the governing equations is used. The velocity profile is obtained by adapting an exact solution to the momentum equation. The energy transport is modeled without invoking the assumption of local thermal equilibrium. Models for the thermal dispersion conductivity, k d , and the interstitial heat transfer coefficient, h sf , are postulated based on physical arguments. The empirical constants in these models are determined by matching the numerical results with the experimental data obtained in this study as well as those in the open literature. Excellent agreement is achieved in the entire range of the parameters studied, indicating that the proposed treatment is sufficient to model forced convection in metal foams for most practical applications

Midstream Modulation of Technology: Governance From Within

Public “upstream engagement” and other approaches to the social control of technology are currently receiving international attention in policy discourses around emerging technologies such as nanotechnology. To the extent that such approaches hold implications for research and development (R&D) activities, the distinct participation of scientists and engineers is required. The capacity of technoscientists to broaden the influences on R&D activities, however, implies that they conduct R&D differently. This article discusses the possibility for more reflexive participation by scientists and engineers in the internal governance of technology development. It reviews various historical attempts to govern technoscience and introduces the concept of midstream modulation, through which scientists and engineers, ideally in concert with others, bring societal considerations to bear on their work.

Non-Darcy natural convection in high porosity metal foams

We present numerical and experimental results for buoyancy-induced flows in high porosity metal foams heated from below. A Brinkman–Forchheimer-extended Darcy flow model and a semi-heuristic two-equation energy model obtained by relaxing the local thermal equilibrium (LTE) assumption are adopted. Experiments conducted under natural convection conditions for the same configuration are used to test the numerical model and the validity of the thermal equilibrium assumption for metal foams. Aluminum foam samples of different pore sizes (5–40 PPI) and porosities (0:89 6 e 6 0:97) are used to illustrate the effects of metal foam geometry on heat transfer. In addition, several metal foam–fluid combinations (aluminum–air, carbon–air, aluminum–water, and nickel–water) are used to study the heat transfer enhancement relative to the base case in which there is no metal foam but only a heated plate. Thermal dispersion effects and the effects of Darcy number on heat transfer are reported. Our results indicate that the thermal non-equilibrium model provides a superior description of heat transfer in metal foams, especially in the presence of fluid–porous interfaces. 2002 Elsevier Science Ltd. All rights reserved.

Temperature dependence of thermal conductivity of biological tissues

In this paper, we present our experimental results on the determination of the thermal conductivity of biological tissues using a transient technique based on the principles of the cylindrical hot-wire method. A novel, 1.45 mm diameter, 50 mm long hot-wire probe was deployed. Initial measurements were made on sponge, gelatin and Styrofoam insulation to test the accuracy of the probe. Subsequent experiments conducted on sheep collagen in the range of 25 degrees C < T < 55 degrees C showed the thermal conductivity to be a linear function of temperature. Further, these changes in the thermal conductivity were found to be reversible. However, when the tissue was heated beyond 55 degrees C, irreversible changes in thermal conductivity were observed. Similar experiments were also conducted for determining the thermal conductivity of cow liver. In this case, the irreversible effects were found to set in much later at around 90 degrees C. Below this temperature, in the range of 25 degrees C < T < 90 degrees C, the thermal conductivity, as for sheep collagen, varied linearly with temperature. In the second part of our study, in vivo measurements were taken on the different organs of a living pig. Comparison with reported values for dead tissues shows the thermal conductivities of living organs to be higher, indicating thereby the dominant role played by blood perfusion in enhancing the net heat transfer in living tissues. The degree of enhancement is different in different organs and shows a direct dependence on the blood flow rate.

A classifier based on the artificial neural network approach for cardiologic auscultation in pediatrics

OBJECTIVE This research work was aimed at developing a reliable screening device for diagnosis of heart murmurs in pediatrics. This is a significant problem in pediatric cardiology because of the high rate of incidence of heart murmurs in this population (reportedly 77-95%), of which only a small fraction arises from congenital heart disease. The screening devices currently available (e.g. chest X-ray, electrocardiogram, etc.) suffer from poor sensitivity and specificity in detecting congenital heart disease. Thus, patients with heart murmurs today are frequently assessed by consultation as well with advanced imaging techniques. The most prominent among these is echocardiography. However, echocardiography is expensive and is usually only available in healthcare centers in major cities. Thus, for patients being evaluated with a heart murmur, developing a more accurate screening device is vital to efforts in reducing health care costs. METHODS AND MATERIAL The data set was collected from incoming pediatrics at the cardiology clinic of The Childrens Hospital (Denver, Colorado), on whom echocardiography had been performed to identify congenital heart disease. Recordings of approximately 10-15s duration were made at 44,100Hz and the average record length was approximately 60,000 points. The best three cycles with respect to signal quality sounds were extracted from the original recording. The resulting data comprised 241 examples, of which 88 were examples of innocent murmurs and 153 were examples of pathological murmurs. The selected phonocardiograms were subject to the digital signal processing (DSP) technique of fast Fourier transform (FFT) to extract the energy spectrum in frequency domain. The spectral range was 0-300Hz at a resolution of 1Hz. The processed signals were used to develop statistical classifiers and a classifier based on our in-house artificial neural network (ANN) software. For the latter, we also tried enhancements to the basic ANN scheme. These included a method for setting the decision-threshold and a scheme for consensus-based decision by a committee of experts. RESULTS Of the different classifiers tested, the ANN-based classifier performed the best. With this classifier, we were able to achieve classification accuracy of 83% sensitivity and 90% specificity in discriminating between innocent and pathological heart murmurs. For the problem of discrimination between innocent murmurs and murmurs of the ventricular septal defect (VSD), the accuracy was higher, with sensitivity of 90% and specificity of 93%. CONCLUSIONS An ANN-based approach for detection and identification of congenital heart disease in pediatrics from heart murmurs can result in an accurate screening device. Considering that only a simple feature set was used for classification, the results are very encouraging and point out the need for further development using improved feature set with more potent diagnostic variables.

Toward a self-deploying shape memory polymer neuronal electrode

The widespread application of neuronal probes for chronic recording of brain activity and functional stimulation has been slow to develop partially due to long-term biocompatibility problems with existing metallic and ceramic probes and the tissue damage caused during probe insertion. Stiff probes are easily inserted into soft brain tissue but cause astrocytic scars that become insulating sheaths between electrodes and neurons. In this communication, we explore the feasibility of a new approach to the composition and implantation of chronic electrode arrays. We demonstrate that softer polymer-based probes can be inserted into the olfactory bulb of a mouse and that slow insertion of the probes reduces astrocytic scarring. We further present the development of a micromachined shape memory polymer probe, which provides a vehicle to self-deploy an electrode at suitably slow rates and which can provide sufficient force to penetrate the brain. The deployment rate and composition of shape memory polymer probes can be tailored by polymer chemistry and actuator design. We conclude that it is feasible to fabricate shape memory polymer-based electrodes that would slowly self-implant compliant conductors into the brain, and both decrease initial trauma resulting from implantation and enhance long-term biocompatibility for long-term neuronal measurement and stimulation.

Contradictory intent? US federal legislation on integrating societal concerns into nanotechnology research and development

This paper argues that the 21st Century Nanotechnology Research and Development (RD and mounting pressure to conduct technology development with more effective regard to societal considerations. The tension emerges when comparing various ‘Program Activities’ set forth in the Act that require divergent policy models, by which the legislation attempts to balance international competition with concern over the perceived risks of nanotechnology applications. By prescribing the integration of societal and technical concerns during nanotechnology R&D, the Act could mark a radical shift in S&T policy in so far as it allows the consideration of societal concerns to influence technological activities and outcomes. Copyright , Beech Tree Publishing.

Characterization of the RF ablation-induced ‘oven effect’: The importance of background tissue thermal conductivity on tissue heating

Purpose: To determine the effect of background tissue thermal conductivity on RF ablation heating using ex vivo agar phantoms and computer modelling. Method: Two-compartment cylindrical agar phantom models (5% agar, 5% NaCl, 3% sucrose) were constructed. These included a standardized inner compartment (2 cm diameter, 4 cm length, 0.25% agar) representing a tumour, surrounded by an outer compartment representing background tissue. The thermal conductivity of the outer compartment was varied from 0.48 W m−1°C (normal liver) to 0.23 W m−1°C (fat) by adding a fat-saturated oil-based solute (10–90%) to the agar. RF ablation was applied at 2000 mA current for 2 min. Temperatures were recorded up to 4 cm from the electrode tip at 1 cm intervals. Subsequently, a 2-D finite element computer model was used to simulate RF ablation of 2–24 min duration for tumours measuring 2–4 cm in diameter surrounded by tissues of different thermal conductivity with the presence or absence of perfusion (0–5 kg m−3 s−1) (n = 44). A comparison of results was performed. Results: In agar phantoms, the amount of fat in the background tissue correlated with thermal conductivity as a negative exponential function (r2 = 0.98). Significantly increased temperatures were observed at the edge of the inner compartment (1 cm from the electrode tip) as the fat content of the outer compartment increased (p < 0.01). Thus, temperatures at 2 min measured 31.5 ± 2.2°C vs 45.1 ± 3.1°C for thermal conductivities of 0.46 W m−1°C (10% fat) and 0.23 W m−1°C (90% fat), respectively. On the other hand, higher levels of fat led to lower temperature increases in the background compartment (0.2 ± 0.3°C for 90% fat vs. 1.1 ± 0.05°C for 10% fat, p < 0.05). Phantom thermal heating patterns correlated extremely well with computer modelling (r2 = 0.93), demonstrating that background tissues with low thermal conductivity increase heating within the central tumour, particularly for longer durations of RF ablation and in smaller tumours. Furthermore, computer modelling demonstrated that increases in temperature at the tumour margin for background tissues of lower thermal conductivity persisted in the presence of perfusion, with a clinically relevant 4.5°C difference between background thermal conductivities of fat and soft tissue for a 3 cm tumour with perfusion of 2 kg m−3 s−1, treated for 12 min. Conclusion: Lower thermal conductivity of background tissues significantly increases temperatures within a defined ablation target. These findings provide insight into the ‘oven effect’ (i.e. increased heating efficacy for tumours surrounded by cirrhotic liver or fat) and highlight the importance of both the tumour and the surrounding tissue characteristics when contemplating RF ablation efficacy.

Metal Foam and Finned Metal Foam Heat Sinks for Electronics Cooling in Buoyancy-Induced Convection

In this paper, we present our recent experimental results on buoyancy-induced convection in aluminum metal foams of different pore densities [corresponding to 5, 10, 20, and 40 pores per in. (PPI)] and porosities (0.89-0.96). The results show that compared to a heated surface, the heat transfer coefficients in these heat sinks are five to six times higher. However, when compared to commercially available heat sinks of similar dimensions, the enhancement is found to be marginal. The experimental results also show that for a given pore size, the heat transfer rate increases with porosity, suggesting the dominant role played by conduction in enhancing heat transfer. On the other hand, if the porosity is held constant, the heat transfer rate is found to be lower at higher pore densities. This can be attributed to the higher permeability with the larger pores, which allows higher entrainment of air through the porous medium. New empirical correlations are proposed for the estimation of Nusselt number in terms of Rayleigh and Darcy numbers. We also report our results on novel finned metal foam heat sinks in natural convection. Experiments were conducted on aluminum foams of 90% porosity with 5 and 20 PPI with one, two, and four aluminum fins inserted in the foam. All of these heat sinks were fabricated in-house. The results show that the finned metal foam heat sinks are superior in thermal performance compared to the normal metal foam and conventional finned heat sinks. The heat transfer increases with an increase in the number of fins. However, the relative enhancement is found to decrease with each additional fin. The indication is that there exists an optimum number of fins beyond which the enhancement in heat transfer, due to increased surface area, is offset by the retarding effect of overlapping thermal boundary layers. Similar to normal metal foams, the 5 PPI samples are found to give higher values of h compared to the 20 PPI samples due to higher permeability of the porous medium. Future work is planned to arrive at the optimal heat sink configuration for even larger enhancement in heat transfer.