Avraham Shitzer

A physiological strain index to evaluate heat stress

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 normothermia to hyperthermia was 3 degrees C (36.5-39.5 degrees C) and 120 beats/min (60-180 beats/min), respectively. Tre and HR were assigned the same weight functions as follows: PSI = 5(Tret - Tre0) . (39.5 - Tre0)-1 + 5(HRt - HR0) . (180 - HR0)-1, where Tret and HRt are simultaneous measurements taken at any time during the exposure and Tre0 and HR0 are the initial measurements. PSI was applied to data obtained from 100 men performing exercise in the heat (40 degrees C, 40% relative humidity; 1.34 m/s at a 2% grade) for 120 min. A separate database representing seven men wearing protective clothing and exercising in hot-dry and hot-wet environmental conditions was applied to test the validity of the present index. PSI differentiated significantly (P < 0.05) between the two climates. This index has the potential to be widely accepted and to serve universally after extending its validity to women and other age groups.

Numerical Solution of the Multidimensional Freezing Problem During Cryosurgery

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 properties, blood perfusion, and metabolic heat generation. The numerical scheme is based on the application of an effective specific heat, substituting the intrinsic property, to include the latent heat effect within the phase transition temperature range. Results of the numerical solution were verified against an existing exact solution of a one-dimensional inverse Stefan problem in Cartesian coordinates. Results were further validated against experimental data available from the literature. The utility of the numerical solution for the design and application of cryodevices is demonstrated by parametric studies of the freezing processes around spherical and cylindrical cryoprobes. The parameters studied are the cryoprobe cooling power and the dimensions of the frozen region. Results are calculated for typical thermophysical properties of soft biological tissues, for angioma and for water.

Experiments with a flat plate solar water heating system in thermosyphonic flow

Abstract 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 m 2 and they were tilted about 35° relative to the horizon. All collector pipes and connecting tubes were made of galvanized steel. The underside collector plate, collector tubes and storage tank were equipped with thermo-couples. A specially designed flow meter was used to measure water flow rate. Results show relatively linear temperature distributions both along the collectors and in the storage tank when no water consumption was allowed. Water flow rate was found to essentially follow solar radiation and reached a maximum of about 950 cm 3 /min. This value was found to be about 33 per cent smaller than the one predicted by an analytical model developed by the authors. It was also observed that shutting the system off during the afternoon hours, when losses to the environment are enhanced, might increase system efficiency.

The natural circulation solar heater-models with linear and nonlinear temperature distributions

L, dimensionless length; L~, overall length of the circulation loop; Q, dimensionless volumetric flow rate; q, dimensionless heat flux per unit length; q0, dimensionless solar radiation heat flux per unit length, absorbed in the collector plate; s, dimensionless coordinate along the circulation loop; T, dimensionless temperature above the ambient; T , dimensionless highest temperature in the system [T~(L~)]; Tin, dimensionless mean temperature of the system; 7~,.~, maximum possible temperature; AT, dimensionless temperature difference along the collector and the tank; U, overall heat-transfer coefficient (per unit length); UL, U, = ^ ~, dimensmnless overall heat-transfer pcpA V coefficient;

Heating of biological tissue by laser irradiation: theoretical model

A mathematical model has been developed for calculating the heating effects of a laser beam on biological tissue. 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 describe the model and discuss its use for calculating temperature fields in lased tissue. Experimentally, we measured the temperature profiles in soft and hard tissue under nonablating conditions. The experimental results are in good agreement with the theoretical ones.

Numerical Analysis of an Extremity in a Cold Environment Including Countercurrent Arterio-Venous Heat Exchange

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 insulation, depicting thermal handwear, is also considered. The tissue is subdivided into four concentric layers simulating, from the center outward, core, muscle, fat, and skin. Differential heat balance equations are formulated for the tissue and for the major artery and the major vein traversing the finger. These coupled equations are solved numerically by a finite-difference, alternating direction method employing a Thomas algorithm. The numerical scheme was extensively tested for its stability and convergence. This paper presents the model equations and results of the convergence tests, and shows plots of blood and tissue temperatures along the axis of the model for combinations of parameters including the effect of countercurrent heat exchange between the artery and the vein.

HEAT TRANSFER ANALYSIS OF A FLAT-PLATE SOLAR ENERGY COLLECTOR

Abstract A model is developed for the heat transfer in a flat plate solar collector with a rectangular channel for water or air flow. This 2-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. Thermal boundary layer development is investigated. Overall efficiencies are calculated for uniform solar heat influx with variable heat losses from the plate. The thermosyphonic effect, due to natural convection, is evaluated and the collectors geometry optimized with respect to this effect.

Analysis of the Inverse Problem of Freezing and Thawing of a Binary Solution During Cryosurgical Processes

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 imposed at the lower temperature boundary of the freezing/thawing front. This condition is believed to be essential for maximizing cell destruction rate. The integral solution yields a temperature forcing function which is applied at the surface of the cryoprobe. An average thermal conductivity, on both sides of the freezing front, is used to improve the solution. A two-dimensional, axisymmetric finite element code is used to calculate cooling/warming rates at positions in the medium away from the axis of symmetry of the cryoprobe. It was shown that these cooling/warming rates were always lower than the prescribed rate assumed in the one-dimensional solution. Thus, similar, or even higher, cell destruction rates may be expected in the medium consistent with existing in vitro data. Certain problems associated with the control of the warming rate during the melting stage are discussed.

Finite Element Analysis of the Temperature Field Around Two Adjacent Cryo-Probes

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 problem yielded reasonable results. The program simulated the nonsymmetric activation of two probes and the merging of the two freezing fronts in the case of symmetric activation.

Controlled Freezing of Nonideal Solutions With Application to Cryosurgical Processes

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 cooling rate during freezing by accurately controlling the probe temperature variation with time. The new device has been experimentally tested by applying it to an aqueous solution of mashed potatoes. The temperature field in the freezing medium, whose thermal properties are similar to those of biological tissue, was measured. The cryoprobe temperature was controlled according to a desired time varying profile which was assumed to maximize necrosis. The tracking accuracy and the stability of the closed loop control system were investigated. It was found that for most of the time the tracking accuracy was excellent and the error between the measured probe temperature and the desired set point is within +/- 0.4 degrees C. However, noticeable deviations from the set point occurred due to the supercooling phenomenon or due to the instability of the liquid nitrogen boiling regime in the cryoprobe. The experimental results were compared to those obtained by a finite elements program and very good agreement was obtained. The deviation between the two data sets seems to be mainly due to errors in positioning of the thermocouple junctions in the medium.