Avraham Shitzer

Technion – Israel Institute of Technology | technion · Faculty of Mechanical Engineering

Space exploration, and particularly extravehicular activities, require creation of a “micro-climate” for the removal of metabolic heat. This is accomplished by a specially designed thermal garment. The early garments used by NASA for the Gemini program were air-ventilated. These proved to be inadequate and were subsequently replaced by water-air hy...

One of the current methods of treating malignant tissues is by localized freezing in situ. Large blood vessels traversing or situated proximal to the treated region, present a “thermal perturbation” to the developing temperature field. This may prevent certain areas of the tumor from being frozen. Alternatively, there is also the problem relating t...

The effects of wind blowing on exposed body parts in cold weather are quite pronounced. They can be manifested either as just a feeling of increased cold‐related discomfort or may even cause frostbite and freeze damages under certain conditions. Quantification of these effects depends on a host of parameters, including air temperature, wind velocit...

Blood pressure drop and arterial distensibility are important, allegedly independent, parameters that are indicative of the coronary arteries patency and atherosclerosis severity. In the present study we show that these two parameters are dependent, allowing to obtain both parameters from a single measurement, and to do so a high-resolution differe...

Coronary artery pressure-drop and distensibility (compliance) are two major, seemingly unrelated, parameters in the cardiovascular clinical setting, which are indicative of coronary arteries patency and atherosclerosis severity. While pressure drop is related to flow, and therefore serves as a functional indicator of a stenosis severity, the arteri...

This study presents an efficient, fast and accurate method for estimating the two-dimensional temperature distributions around multiple cryo-surgical probes. The identical probes are inserted into the same depth and are operated simultaneously and uniformly. The first step in this method involves numerical derivation of the temporal performance dat...

The assignments of basal metabolic rates (BMR), basal cardiac output (BCO), and basal blood perfusion rates (BBPR) were compared in nine multi-compartment, whole-body thermoregulation models. The data are presented at three levels of detail: total body, specific body regions, and regional body tissue layers. Differences in the assignment of these q...

This article is a reply to comments by R. Osczevski on "Modified wind chill temperatures determined by a whole body thermoregulation model and human-based convective coefficients."

The advantage of measuring differential pressure using fluid-filled catheters is that the system is relatively inexpensive, but the readings are not accurate and affected by the common mode pressure (CMP) distortion. High accuracy differential pressure measurements are required in various biomedical applications, such as in fluid-dynamic test rigs,...

Wind chill equivalent temperatures (WCETs) were estimated by a modified Fiala's whole body thermoregulation model of a clothed person. Facial convective heat exchange coefficients applied in the computations concurrently with environmental radiation effects were taken from a recently derived human-based correlation. Apart from these, the analysis f...

This study presents an optimization procedure for the design of a Latent Heat Thermal Management System (LHTMS), used for cooling an electronic device with transient and high heat generation. The LHTMS consists of Phase Change Material (PCM) combined with internal fins, which are used for creating high conductive paths into the PCM. The optimizatio...

The most accepted standard tool of cardiologists for assessing stenoses severity in the coronary tree is coronary angiography. Information from coronary angiography is limited to geometrical data on the coronary lumen, and provides limited functional data on the severity of stenoses. In addition, histopathological studies have demonstrated that ang...

Facial heat exchange convection coefficients were estimated from experimental data in cold and windy ambient conditions applicable to wind chill calculations. Measured facial temperature datasets, that were made available to this study, originated from 3 separate studies involving 18 male and 6 female subjects. Most of these data were for a −10°C a...

This article presents a retrospective of work performed at the Technion, Israel Institute of Technology, over the last 3-odd decades. Results of analytical and numerical studies are presented briefly as well as in vitro and in vivo experimental data and their comparison to the derived results. Studies include the analysis of both the direct (Stefan...

This article discusses briefly the advances made and the remaining short-comings in the "new" wind chill charts adopted in the US and Canada in 2001. A number of indicated refinements are proposed, including the use of whole body models in the computations, verification of heat exchange coefficients by human experiments, reconsideration of "calm" w...

Fifty-two experimental subjects (50% female) were seated in a climate chamber and exposed to operative temperature ramps with different slopes, directions, and durations during two related experiments. The first experiment covered a temperature range of 22°C–26.8°C (71.6°F–80.2°F) and subjects wore light clothing (0.5 clo). The operative temperatur...

The dynamic thermal interaction between a surface cryosurgical probe (heat sink) and an embedded cylindrical tube (heat source), simulating a thermally-significant blood vessel, has been studied. The cryoprobe was operated by liquid nitrogen while the embedded tube was perfused by water at a constant inlet temperature. Previous experimental data we...

The growing need for valid assessment procedures of the outdoor thermal environment in the fields of public weather services, public health systems, urban planning, tourism & recreation and climate impact research raised the idea to develop the Universal Thermal Climate Index UTCI based on the most recent scientific progress both in thermo-physiolo...

One, two, and three needle cryoprobes, 1.47 mm outside diameter simultaneously and uniformly operated by high pressure argon gas, were tested in a gel simulating the thermal properties of biological tissues. The probes were inserted into the same depth in the gel through two parallel templates with holes drilled on a 5 × 5 mm2 mesh. The temperature...

A transient analysis of the human-environment thermal interaction in cold and windy environments is presented. The site selected to represent this interaction is the head-face, which is depicted as a hollow cylinder wherein heat is conducted in the radial direction. Environmental radiation effects, which are small relative to the wind-driven convec...

Combinations of wind-driven convection and environmental radiation in cold weather, make the environment "feel" colder. The relative contributions of these mechanisms, which form the basis for estimating wind chill equivalent temperatures (WCETs), are studied over a wide range of environmental conditions. Distinction is made between direct solar ra...

Three-dimensional numerical simulations of multi-cryo-needle surgery were performed with cryo-needle temperature variations taken from matched experimental data. The transient temperatures and frozen volumes generated by simultaneously operating up to three 1.47 mm OD cryo-needles embedded in a phase-changing gel simulating the properties of biolog...

A model of facial heat exchange in cold and windy environments is presented. The tissue is depicted as a hollow cylinder and the model includes heat conduction and heat transport by blood circulation from the warmer core. A steady-state solution facilitating the estimation of wind chill equivalent temperature (WCET) as a function of the effective w...

An apparent error was detected in the calculation of windchill equivalent temperatures (WCETs) in the ``new'' chart and corresponding equation that were adopted in 2001 by the weather services in the United States and Canada. The problem is caused by significant discontinuities in WCETs at the assumed ``calm'' wind speed condition of 1.34 m s-1. As...

The wind-chill index (WCI), developed in Antarctica in the 1940s and recently updated by the weather services in the USA and Canada, expresses the enhancement of heat loss in cold climates from exposed body parts, e.g., face, due to wind. The index provides a simple and practical means for assessing the thermal effects of wind on humans outdoors. I...

A first order analytical approximation of steady-state heat conduction in a hollow cylinder exchanging heat at its external surface by convection with a cold and windy environment is presented. The model depicts the thermal behavior of certain body elements, e.g., head/face, when exposed to such environments. The results are presented by dimensionl...

The effects of a thermally-significant blood vessel, simulated by an embedded acrylic tube, 4.8 mm outer diameter on the freezing field caused by a surface cryoprobe were studied experimentally in a tissue phantom. The flat, 15 mm diameter, circular cryoprobe was operated at a constant cooling rate of -8 degrees C/min by liquid nitrogen down to -60...

Comparison of two commonly used calculation methods of the heat of fusion of food products yielded an unexpected fourfold difference between the results. The sources for this disparity are investigated and resolved by proposing amendments to equations presented in Chapter 8 of the 2002 ASHRAE Handbook—Refrigeration. Adjustment to a common zero enth...

The relative importance of skin vs. core temperature for stimulating cold acclimation (CA) was examined by 5 wk of daily 1-h water immersions (20 degrees C) in resting (RG) and exercising (EG) subjects. Rectal temperature fell (0.8 degrees C; P < 0.05) during immersion only in RG. Skin temperature fell (P < 0.05) similarly in both groups. Physiolog...

Thermal efficiency of cold-stressed finger-tips during cold induced vasodilatation (CIVD) is considered. The actual heat loss from the finger-tip is compared to either the minimal or the maximal heat losses. The actual heat loss is estimated by integrating the area under the time-temperature curve of the finger-tip. The minimal heat loss is estimat...

A physiological strain index to evaluate heat stress You might find this additional info useful... 17 articles, 6 of which you can access for free at: This article cites http://ajpregu.physiology.org/content/275/1/R129.full#ref-list-1 18 other HighWire-hosted articles: This article has been cited by http://ajpregu.physiology.org/content/275/1/R129#...

Seven gloves were studied worn by eight sedentary subjects (six men and two women) exposed to cold-dry, C D, (mean dry bulb temperature Tdb = -17.2 degrees C; mean dew point temperature Tdp = -25.1 degrees C), and cold-wet, C-W, (Tdh = 0 degrees C; Tdp = -8.4 degrees C) conditions. Mean endurance times were 75 min for the C-D and 162 min for the C...

The thermal response of fingers exposed to cold weather conditions has been simulated. Energy balance equations were formulated, in a former study, for the tissue layers and the arterial, venous, and capillary blood vessels. The equations were solved by a finite difference scheme using the Thomas algorithm and the method of alternating directions....

A physiological strain index (PSI), based on rectal temperature (Tre) and heart rate (HR), capable of indicating heat strain online and analyzing existing databases, has been developed. The index rates the physiological strain on a universal scale of 0-10. It was assumed that the maximal Tre and HR rise during exposure to exercise heat stress from...

A multidimensional, finite difference numerical scheme for the freezing process of biological tissues during cryosurgery is presented, which is a modification of an earlier numerical solution for inanimate materials. The tissues are treated as nonideal materials, freezing over a temperature range and possessing temperature-dependent thermophysical...

The relative importance of core vs. skin temperature reduction as a stimulus for cold acclimation (CA), was assessed by evaluating thermoregulatory responses during cold air exposure (90 min, 5 deg C; CAE), cold water immersion (60 min, 20 deg C; CWI), and cold finger immersion (30 min, 4 deg C; CFI), before and after CA. Subjects acclimated for fi...

A new combined solution of the one-dimensional inverse Stefan problem in biological tissues is presented. The tissue is assumed to be a nonideal material in which phase transition occurs over a temperature range. The solution includes the thermal effects of blood perfusion and metabolic heat generation. The analysis combines a heat balance integral...

A model of the thermal behavior of an extremity, e.g., a finger, is presented. The model includes the effects of heat conduction, metabolic heat generation, heat transport by blood perfusion, heat exchange between the tissue and the large blood vessels, and arterio-venous heat exchange. Heat exchange with the environment through a layer of thermal...

Simultaneous measurements of middle finger-tip temperatures and blood perfusion rates were made in hands exposed to 0°C air. Temperatures were measured by a thermocouple disk attached to the nail-bed of the finger. Blood perfusion was monitored by a laser-Doppler velocimeter probe attached to the palmar side of the finger-tip. Seven, 60–80 min long...

Measured cold induced vasodilatation (CIVD) temperature and pulse volume changes were used to validate a lumped-parameter heat exchange model of the finger-tip. The simultaneously measured data were extracted from a previous study by Blaisdell (1951).Finger-tip temperatures were estimated using adjusted blood perfusion data as an input. The model w...

A lumped-parameter model of a fingertip is presented. The semispherical model includes the effects of heat storage, heat exchange with the environment, and heat transport by blood perfusion. The thermal insulation on the surface of the fingertip is represented by the overall heat transfer coefficient that is calculated by common engineering formula...

A new cryosurgical device utilizing liquid nitrogen, which is a modification of an existing commercial system, was developed. In the new computer-controlled cryodevice the temperature of the cryoprobe is controlled by means of an electrical heating element. The desired temperature-forcing function is calculated to ensure a specified constant coolin...

A new cryosurgical device was developed in this study to facilitate examination of factors affecting the outcome of cryotreatment. Special emphasis was placed on the control of the cooling rate at the freezing front. In the new computer-controlled cryosurgical device, the controlling variable is the cryoprobe temperature, which is calculated to ens...

An integral solution for a one-dimensional inverse Stefan problem is presented. Both the freezing and subsequent thawing processes are considered. The medium depicting biological tissues, is a nonideal binary solution wherein phase change occurs over a range of temperatures rather than at a single one. A constant cooling, or warming, rate is impose...

A new analytic solution of the inverse-Stefan problem in biological tissues is presented. The solution, which is based on the enthalpy method, assumes that phase change occurs over a temperature range and includes the thermal effects of metabolic heat generation, blood perfusion, and density changes. As a first stage a quasi-steady-state solution i...

A numerical model of the thermal behavior of an extremity, e.g., finger, is presented. The model includes the effects of: (a) heat conduction (b) metabolic heat generation, (c) heat transport by blood perfusion, (d) heat exchange between the tissue and the large blood vessels, and, (e) arterio-venous heat exchange. Heat exchange with the environmen...

A finite element code was developed for the analysis of the temperature field around two adjacent cylindrical cryo-probes. The two-phase, two-dimensional Stefan problem is solved using a moving boundary approach and space-time finite elements. Solution of one-cryo-probe problem compared well with an existing analytic solution. The two-cryo-probes p...

A mathematical model has been developed for calculating the heating effects of a laser beam on biological tissues. The model may be used for calculating the temperature rise and the extent of damage in tissue exposed to laser irradiation. It can be applied in conjunction with laser coagulation, laser surgery, laser hyperthermia, etc. We briefly des...

A numerical model was developed for calculating the temperature distribution in material exposed to a laser beam. The model takes into account the different stages under undergone by the tissue before material removal is achieved. The consecutive steps considered are: heating to the boiling temperature of water, boiling and evaporation of the water...

The estimation of endurance times of the digits exposed to cold weather is performed by an analytical, one-dimensional cylindrical model. Blood perfusion effects are lumped into a volumetric heat-generation term. Cold-induced vasodilatation (CIVD) effects are not included in the present analysis. Endurance times, defined by a drop in cylinder tip t...

Success of a cryosurgical procedure, i.e., maximal cell destruction, requires that the cooling rate be controlled during the freezing process. Standard cryosurgical devices are not usually designed to perform the required controlled process. In this study, a new cryosurgical device was developed which facilitates the achievement of a specified cool...

A theoretical analysis and experimental results of a controlled cryosurgical process are presented. The analysis is based on the energy equations describing the probe response and the phase change occurring in the medium. The device developed is based on the standard cryosurgical system. consisting of a liquid nitrogen tank, insulated tube, and cry...

The model developed in this study is useful in calculating temperature variations along insulated digits and in predicting their endurance times to cold exposures. Calculated results compared very favorably with data obtained on human subjects. The present version of the model does not include the effect of cold induced vasodilation. Expansion of t...

A model for calculating temperature fields and ablation processes in tissue was developed. The model assumes three different stages experienced by the tissue during a complete ablation process. The consecutive steps are: heating, boiling and ablation. The model has been examined by calculations and experimental measurements of temperature fields in...

To protect the pediatric surgeon from heat stress while operating on premature infants under an overhead radiant heater, we propose covering the surgeon's head with an ordinary straw hat wrapped with heavy-duty kitchen aluminum foil. The temperature under such a straw hat is approximately 37 degrees C, versus 52 degrees C under a surgical cotton ca...

A solar air heating system is designed for a floor of 120 m2 offices, with large thermal capacity walls, in Israel. A constant air volume system is chosen for its operational simplicity. Representative winter hourly weather data are used to calculate the heating load. The building behavior is modeled in detail with dynamic wall and room temperature...

The temperature fields around cryoprobes were investigated analytically and experimentally. Two cryoprobes were employed: a spherically shaped general purpose probe utilizing liquid nitrogen and a cylindrical "glaucoma" probe utilizing the Joule-Thomson effect in gaseous CO2. Both probes were operated by commercial cryostats. The analytical solutio...

One of the important tasks in physiology and medicine is estimating tissue blood perfusion rate and heat generation rate due to metabolism and other sources. This information is required for experimental studies as well as for clinical purposes and has been the subject of numerous ingenious efforts. The difficulties in performing these measurements...

Several ingenious methods are currently used for tissue removal and destruction, including techniques based on mechanical, chemical, and physical principles. The usual mechanical method involves a cutting knife—the “scalpel.” The chemical method is based on the introduction of suitable chemical agents, systemically or locally, to achieve the desire...

An analytical solution to the transient thermal behavior of a rock-bed thermal storage system subjected to varying air inlet temperature is presented. The solution is obtained in terms of infinite double series which are based on Schumann’s solution, derived for a step change in air inlet temperature. In the present study, bed and fluid temperature...

A simple mathematical algorithm which facilitates the design of a latent heat reservoir for stabilizing the temperature of an inflight electronic device is developed, and the behavior of paraffin wax in a heat storage capacitor is experimentally studied. The results show that in the solidification stage the heat transfer coefficient h is smaller th...

One of the most important tasks in physiology and medicine is the estimation of tissue blood perfusion rate and heat generation rate due to metabolism and other sources. This information is required for experimental studies as well as for clinical purposes and has been the subject of numerous ingenious efforts. The difficulties in performing these...

A computerized simulation and design of solar operated NH3H2O absorption refrigeration cycles is presented. The program receives input data and calculates the building, cooling and heating loads initially. Next the absorption cycle is designed along with the solar collectors and auxiliaries. Various economic parameters are also calculated from whi...

Numerical methods and the bio-heat transfer equation are employed to calculate temperature profiles in tissues subjected to nonuniform blood flow distribution for initial and boundary conditions which simulate experimental physiological situations. Results indicate that one can infer, from sudden changes in temperature distribution, the occurrence...

Thermodilution (heat clearance) was introduced 50 years ago for prediction of blood flow rate in tissue. The method lost favour because of the size of the temperature probes, technical requirements for generation of a tissue temperature perturbation, and lack of appreciation of interplay of conduction, heat storage, and convective transport. Recent...

Recording the washout of indicator (for example, heat, radio-labeled dissolved gas, etc.) transiently introduced into tissue allows the estimation of tissue blood perfusion rate. Analysis of the washout data requires a material balance which appropriately accounts for all transport mechanisms and sources and sinks of the given indicator. From that...

A general, one-dimensional steady-state analysis of the thermal behavior of biological tissues is presented. The three major geometries, i.e., rectangular, cylindrical and spherical, are considered along with boundary conditions of the general type. It is shown that transport of heat inside the tissue may be dominated by conduction, convection by b...

An analysis of the temperature fields developed in a biological tissue undergoing a monoactive electrical coagulating process is presented, including thermal recovery following prolonged heating. The analysis is performed for the passage of alternating current and assumes a homogeneous and isotropic tissue model which is uniformly perfused by blood...

The sensitivity of indicator dilution measurement of capillary perfusion rate to errors in the experimental variables was studied. Parameter sensitivity analysis and models based on a diffusion equation with a capillary-level convection term were used. The sensitivity analysis is based on normalized partial derivatives of indicator concentration wi...

A typical Israeli water heating system in thermosyphonic-flow was tested. The system consisted of two flat plate collectors painted matt black connected in parallel and a 140 l. storage tank. Total surface area of the collectors, employing the parallel flow pattern, was about 3 m2 and they were tilted about 35° relative to the horizon. All collecto...

Recording the washout of indicator (for example, heat, ratio-labeled dissolved gas, etc. ) transiently introduced into tissue allows the estimation of tissue blood perfusion rate. Analysis of the washout data requires a material balance which appropriately accounts for all transport mechanisms and sources and sinks of the given idicator. From that...

Long time analytic solutions to the inverse Stefan problem are presented in Cartesian and spherical coordinates. This problem deals with the phase change of a substance with the boundary conditions specified at the moving boundary. The solutions, expressed in infinite series, facilitate the calculation of temperature and heat flux variations in the...

Three experiments were run with each of five young male subjects to investigate the effects of three different environments on the recovery from heat stress. Duration of all the experiments was 2 h. The first hour was spent on a bicycle ergometer working at a load of about 60W in a 40°C and about 25 % RH chamber. In the second hour, subjects were e...

A theoretical discussion is presented of the stability characteristics of the free convection loop representing the thermosyphonic solar water heater. The system consists of a flat-plate collector, a storage tank and connecting pipes. A one-dimensional model is employed with only one space-coordinate running along the circulation loop. It is assume...

A model is developed for the heat transfer in a flat-plate solar collector with a rectangular channel for water or air flow. This two-dimensional geometry offers the maximum area of contact between the fluid and the collecting surface exposed to the sun. The analysis yields temperature and heat-flow distributions in both dimensions of the collector...

An analytical solution is obtained to the one-dimensional bio-heat equation in Cartesian coordinates. This solution is compared with experimental values obtained inside the quadriceps of exercising human subjects. Blood perfusion and metabolic heat generation rates, and skin surface heat fluxes, are estimated by a least-squares fit. Time constants...

The purpose of the study described is to investigate the feasibility of operating the solar absorption system for space cooling and as a heat pump for space heating. The approach permits the year-round utilization of the rather expensive system of solar collectors which has to be installed in any case. The thermodynamic analysis is performed for va...

Analysis of a Stefan-like problem in the in vivo freezing of a biological tissue is presented in cartesian coordinates. The analysis allows the inclusion of blood perfusion, metabolic heat and tissue heat capacity. Solutions are obtained for the temperature distributions in the frozen and unfrozen regions at different times. A constant freezing rat...

A general analysis is presented for the thermal behavior of a biological tissue. Energy transport by the circulatory system is assumed to be represented by a modified Fick's law. General boundary conditions are assumed for the two-dimensional model and solutions are obtained for rectangular, cylindrical, and spherical geometries. The effects of blo...

A modular liquid cooled helmet liner made of 8 form fitting neoprene patches was constructed. The liner was integrated into the sweatband of an Army SPH 4 helicopter aircrew helmet. This assembly was tested on 4 subjects seated in a hot (47°C), humid (40%) environment. Results indicate a marked reduction in the rate of increase of physiological bod...

A steady-state analytical method has been developed to estimate the amount of heat extracted from a blood vessel running close to the skin surface which is cooled in a symmetrical fashion by a cooling strip. The results indicate that the optimum width of a cooling strip is approximately three times the depth to the centerline of the blood vessel. T...

Tests were performed on 12 cooling patches of various designs to establish criteria for the evaluation of their performance in liquid-cooled suits in industrial, military and aerospace applications. The thermal effectiveness value was 0.088 for patch designs with a double spiral flow pattern, and 0.075 for patch designs with a parallel flow pattern...

This short communication is intended to supplement the list of references included in a previous review on the subject.

A water cooled garment was constructed and used to study the characteristics of independent regional cooling of the body in contrast to the current practice of uniform cooling. The cooling pads in the garment were grouped to provide independent control of water inlet temperatures and flow rates to six regions: head, upper torso, lower torso, arms,...