Thermal Clearance Research Articles

The Relative Performance of External Casing Impingement Cooling Arrangements for Thermal Control of Blade Tip Clearance

As a key way of improving jet engine performance, a thermal tip clearance control system provides a robust means of manipulating the closure between the casing and the rotating blade tips, reducing undesirable tip leakage flows. This may be achieved using an impingement cooling scheme on the external casing. Such systems can be optimized to increase the contraction capability for a given casing cooling flow. Typically, this is achieved by changing the cooled area and local casing features, such as the external flanges or the external cooling geometry. This paper reports the effectiveness of a range of impingement cooling arrangements in typical engine casing closure system. The effects of jet-to-jet pitch, number of jets, and inline and staggered alignment of jets on an engine representative casing geometry are assessed through comparison of the convective heat transfer coefficient distributions as well as the thermal closure at the point of the casing liner attachment. The investigation is primarily numerical, however, a baseline case has been validated experimentally in tests using a transient liquid crystal technique. Steady numerical simulations using the realizable k–ε, k–ω SST, and EARSM turbulence models were conducted to understand the variation in the predicted local heat transfer coefficient distribution. A constant mass flow rate was used as a constraint at each engine condition, approximately corresponding to a constant feed pressure when the manifold exit area is constant. Sets of local heat transfer coefficient data generated using a consistent modeling approach were then used to create reduced order distributions of the local cooling. These were used in a thermomechanical model to predict the casing closure at engine representative operating conditions.

Comparison between ambulatory measurement of effective thermal conductivity and laser Doppler flowmetry method to assess skin microcirculatory activity

The main objective of this paper was to assess the performance of the ambulatory device μHematron to measure indirectly skin blood flow relative to the well-established Laser Doppler flowmetry method. The μHematron device is dedicated to the non-invasive measurement of effective thermal conductivity of living tissues, based on the thermal clearance method. Its major advantage is its ambulatory functionality, as available methods for evaluation of microcirculatory activity are non-ambulatory methods. An experiment was conducted on ten healthy women exposed for one hour in three different thermal environments (22°C, 25°C and 30°C). Skin microcirculatory activity was analyzed after an acclimatization period of 30minutes. The time between each exposure was at least one hour. Performances of the μHematron device were assessed and a comparative study with a laser Doppler perfusion monitor (LDPM) was performed. Good correlation coefficients between the two devices (r=0.71 at T1=22°C, r=0.77 at T2=25°C and r=0.83 at T3=30°C) were obtained while the LDPM signal was filtered by a low pass filter (0.1Hz). These results showed that continuous monitoring of effective thermal conductivity was possible in neutral and warm ambiences. Then, the μHematron device could be considered as a complementary tool to Doppler techniques for the investigation of skin blood flow, when ambulatory conditions are required.

Photoacoustic thermal diffusion flowmetry

Thermal Diffusion Flowmetry (TDF) (also called Heat Clearance Method or Thermal Clearance Method) is a longstanding technique for measuring blood flow or blood perfusion in living tissues. Typically, temperature transients and/or gradients are induced in a volume of interest and the temporal and/or spatial temperature variations which follow are measured and used for calculation of the flow. In this work a new method for implementing TDF is studied theoretically and experimentally. The heat deposition which is required for TDF is implemented photothermally (PT) and the measurement of the induced temperature variations is done by photoacoustic (PA) thermometry. Both excitation light beams (the PT and the PA) are produced by directly modulated 830 nm laser diodes and are conveniently delivered to the volume under test by the same optical fiber. The method was tested experimentally using a blood-filled phantom vessel and the results were compared with a theoretical prediction based on the heat and the photoacoustic equations. The fitting of a simplified lumped thermal model to the experimental data yielded estimated values of the blood velocity at different flow rates. By combining additional optical sources at different wavelengths it will be possible to utilize the method for non-invasive simultaneous measurement of blood flow and oxygen saturation using a single fiber probe.

Optimal Design on Clearance of Piston Friction Pair for Water Hydraulic Pump

The optimal clearance formula of piston friction pair for water hydraulic pump was established based on thermal balance principle. Simulations and experimental research were carried out on its influence factors, results show that, a minimum thermal clearance exists in each of the piston friction pair and its initial clearance must be greater than this one, otherwise it will happen to death. Some effective measures can take to reduce the minimum thermal balance clearance as follows: decreasing the piston diameter or swash-plate tilt angle, choosing the appropriate couple material, reducing friction coefficient and fully running-in, etc.

Simultaneous measurements of local tissue temperature and blood perfusion rate in the canine prostate during radio frequency thermal therapy

Local tissue temperature and blood perfusion rate were measured simultaneously to study thermoregulation in the canine prostate during transurethral radio-frequency (RF) thermal therapy. Thermistor bead microprobes measured interstitial temperatures and a thermal clearance method measured the prostatic blood perfusion rate under both normal and hyperthermic conditions. Increase in local tissue temperature induced by the RF heating increased blood perfusion throughout the entirety of most prostates. The onset of the initial increase in blood perfusion was sometimes triggered by a temporal temperature gradient at low tissue temperatures. When tissue temperature was higher than 41 degrees C, however, the magnitude and the spatial gradient of temperature may play significant roles. It was found that the temperature elevation in response to the RF heating was closely coupled with local blood flow. The resulting decrease in or stabilization of tissue temperature suggested that blood flow might act as a negative feedback of tissue temperature in a closed control system. Results from this experiment provide insights into the regulation of local perfusion under hyperthermia. The information is important for accurate predictions of temperature during transurethral RF thermal therapy.

Simultaneous Recording of Fingertip Skin Blood Flow Changes by Multiprobe Laser Doppler Flowmetry and Frequency-Corrected Thermal Clearance

We compared the results of skin blood flow (SBF) measurements, obtained simultaneously in adjacent fingertips by laser Doppler flowmetry (LDF) and thermal clearance (TC) probes, having approximately the same spatial and temporal characteristics. Experiments were performed in nine healthy volunteers during rest at room temperature (26–28°C). A time resolution equal to about a second was achieved by speeding up the response of the thermal measurement circuit by applying the computer simulation software of the MATLAB package. The comparison revealed that the frequency-corrected TC signal correlated well with the multiprobe LDF signal (median correlation coefficient = 0.90, range = 0.84 to 0.96). At the same time the individual slope values of the regression equation ranged from 0.58 to 1.61, revealing the difficulties encountered in obtaining the invariant scaling factor between the TC and LDF measurements. The relationship between the frequency-corrected TC signal and the multiprobe LDF signal was found to be linear in the range of SBF changes of about three- to fourfold. In the case of larger fluctuations in SBF, excessive acceleration at high SBF rates was noted.

BPA's Probability Based Clearance Buffers, Part III: Application to the Analysis of Existing Lines

A procedure to develop clearance buffers for the analysis of existing lines, based on probabilistic techniques, as used by the Bonneville Power Administration is described. A statistical clearance model for existing lines (the thermal analysis statistical clearance model) tailors the expected scatter in field clearance about a profile clearance for an existing line. This expected field clearance scatter results from measurement tolerances in the design and construction of overhead lines. The paper discusses the use of assurance levels for maintaining regulatory clearance requirements in the field and relates those assurance levels to the statistical clearance model. Using the statistical clearance model and assigned assurance levels, a process is described that establishes the thermal capacity of an existing line based on present regulatory clearance requirements. Finally, an existing line is analyzed to demonstrate the steps to implement the probabilistic clearance analysis process.

Similarity of fingertip skin blood flow patterns recorded by the model-based thermal clearance and large area laser Doppler probes

We compared two different techniques for non-invasive registration of fingertip skin blood flow patterns in 15 healthy volunteers during a rest at room temperature 26–28°C. The large area probes of a two-channel laser Doppler flowmeter (LDF) and an experimental probe of a thermal clearance (TC) instrument provided approximately the same sample volume of the studied tissue. The laser Doppler probes were attached to the pulp of the index (LDF1) and ring (LDF2) fingers, while the TC probe to the pulp of the middle finger. A computer simulation method, provided by the MATLAB package, was introduced to speed up the frequency response of the TC probe. By calculating the correlation coefficients and applying the least squares criterion, we evaluated the similarity of the 3-min samples having time resolution of 1 s. The group-averaged value of the correlation coefficient (median with a 99% confidence interval) for LDF2 versus LDF1 equalled 0.95 (0.92–0.97). After an introduction of the computer-based frequency correction, the correlation coefficient for TC versus LDF1 increased from 0.81 (0.61–0.85) till 0.92 (0.84–0.95), while the root mean squared error between TC and LDF1 diminished by 33% ( p<0.001).

Thermoregulation in the canine prostate during transurethral microwave hyperthermia, part II: Blood flow response

Recent studies on transurethral-applied local hyperthermia in both the canine and human prostate have revealed significant effects of natural thermoregulation on the therapeutical results in the prostate. Using a thermal clearance method, blood perfusion rates were measured within different regions in the canine prostate under normal and hyperthermic conditions. It has been found that the canine prostatic blood perfusion is strongly linked to the geometrical location within the gland, the local tissue temperature, and the imposed thermal dosage. Results from this research are expected to provide a better understanding of the thermoregu latory behaviour in the canine prostate, and thus lay an important foundation for predicting tissue temperature in the human prostate during transurethral microwave hyperthermia.

The response of human tumour blood flow to a fractionated course of thermoradiotherapy

The response of human tumour blood flow to a fractionated course of thermoradiotherapy was documented in four superficial but bulky tumours (three ade—nocarcinomas, one melanoma). Blood flow was measured 15, 30, 45, and 60 min after the onset of heating. These measurements were made at the same intra—tumour point during each heat fraction by use of a modified thermal clearance technique in which a correction was made for the heat dissipated by thermal conduction. This point was at least 2 cm beneath the surface in the central portion of the tumour. Extracellular pH was measured within 1 cm of this point prior to the first heat fraction and 2‐3 weeks later. Hyperthermia was administered for 60 min, twice a week for 4 weeks by use of a 16-channel 915 MHz microwave applicator. Each patient also received a radiation dose of 40 Gy fractionated at 2Gy/fx, five times a week (adenocarcinomas) or 4Gy/fx, twice a week (melanoma). Blood flow remained relatively constant during heating after steady state conditions were attained. However, an overall decrease in tumour blood flow was observed in each patient over the course of thermoradiotherapy. In each case, a relatively small decrease in blood flow occurred between most heat fractions which resulted in an overall decrease which ranged from 50–100% However, there was a tendency for blood flow to increase following the initial heat fraction at points where the steady state temperature was approximately 41 °C or less. Extracellular pH increased in two of three patients and decreased in the other.

Effect of i.v. dipyridamole on cerebral blood flow, blood pressure, plasma adenosine and cAMP levels in rabbits

In response to intravenous administration of dipyridamole, the quantitative and temporal changes in plasma adenosine and cyclic AMP (cAMP) levels in relation to the changes in cerebral blood flow (CBF) and mean arterial blood pressure (MABP) have not been studied. Therefore, we investigated simultaneously the changes in CBF (hydrogen and thermal clearance methods), MABP, plasma adenosine (HPLC) and cAMP (radioimmunoassay) levels for 1 h after intravenous injection of 0.7 and 1.4 mg/kg dipyridamole in rabbits. In separate experiments, only plasma adenosine concentrations were measured to determine how and for how long intravenous administration of 0.7 mg/kg dipyridamole is able to inhibit the removal of plasma adenosine. Dipyridamole decreased MABP, increased plasma adenosine and cAMP levels in a dose-dependent manner. The dose-dependency of increases in CBF could not be demonstrated owing to the marked hypotension. The increase in plasma adenosine concentrations was biphasic. The first peak could be detected at the end of the dipyridamole injection. The second peak occurred 20 min after drug administration, simultaneously with the maximal increases in plasma cAMP level and CBF, whereas the maximal fall in MABP developed earlier. Intravenous administration of 0.7 mg/kg dipyridamole inhibited adenosine uptake only by 25%, which lasted less than 10 min. We concluded that intravenously given dipyridamole is responsible only for the initial short-lasting elevation of plasma adenosine concentration, and is able to induce vasodilation without either dipyridamole itself or adenosine necessarily gaining access to the muscular layer.

Development of noninvasive measurement of peripheral circulation and its medical application

Surveys were carried out on tissue blood flow measurement based on the thermal diffusion method and on the assessment of peripheral circulatory function using photosensors.Regarding the thermal diffusion method, first noninvasive measurement using a Peltier stack was carried out. Then, measurements using a thermal clearance curve at various temperatures were performed.For noninvasive measurement of the mechanical properties of peripheral arteries using photosensors, the author determined the vascular volume ratio and/or the relative vascular volume.For clinical application in field studies, it is necessary to develop an apparatus with which absolute evaluation of the intravascular volume can be carried out using the blood volume around the unit volume as an indicator. Therefore, an apparatus that removes a signal constituent from tissue other than the blood using two-wavelength photosensors is required.If a noninvasive method to accurately measure the intravascular volume is established, changes in the mechanical properties of the peripheral arteries that correspond to functional and organic changes in the normal response can be observed more directly. Moreover, quantitative assessment of peripheral circulatory function will become possible, and diagnosis of the severity of peripheral circulatory disorders will be facilitated.

Clinical Usefulness of Determining the Rate of Thermal Clearance within Heated Tumors

Between June 1987 and June 1988, 28 patients (28 tumors) with liver, retroperitoneal, intrapelvic, or superficial tumors were treated with hyperthermia combined with radiotherapy and/or chemotherapy. Hyperthermia was administered once or twice a week for 30-60 min per session, up to a total of 2-11 sessions, with an 8-MHz RF capacitive heating device. Blood flow in the tumors was evaluated from the rate of thermal clearance (TCR) using the bio-heat transfer equation. The TCR was measured in the middle of the first heating session and at the end of the last heating session by turning off the output power of the heating device. For 9 patients, contrast-enhanced CT scans were taken and CT numbers at the centers of tumors were measured before and after the entire course of hyperthermia. Changes in TCR were closely related to average tumor center temperature, changes in CT number, and tumor response. When smaller and more superficial tumors were treated by hyperthermia combined with radiotherapy and/or chemotherapy that consisted of many heating sessions and during which a high average tumor center temperature was achieved, a better tumor response was obtained. The better the tumor response, the higher the local control rate became. The cause-specific survival rate of patients who achieved good tumor responses was higher than that of patients who showed poor tumor responses. Changes in TCR and CT number in heated tumors were useful and important indicators of tumor response to hyperthermia.

Irradiation induced blood flow changes in the breast during radiotherapy--a pilot study.

Skin blood flow and underskin temperature were measured in breasts during the complementary radiotherapy following lumpectomy, using the noninvasive transient thermal clearance method. On average, blood flow and tissue temperature increased at the start of the irradiation then slightly decreased. The response varied significantly among the patients. The increase in blood flow is associated with the vasodilatation which accompanies irradiation-induced inflammation and the method enables the assessment of this vasodilatation during the irradiation treatment. The decrease of blood flow after the initial increase is attributed to vascular damage. The method provides a noninvasive means for the evaluation of radiation effects on individual patients during radiotherapy.

温熱治療時における温度降下率の臨床的意義に関する検討

温熱治療時における温度降下率の臨床的意義に関する検討

A new approach for predicting the enhancement in the effective conductivity of perfused muscle tissue due to hyperthermia.

This study attempts to measure the hyperthermic response of individual microvessels in skeletal muscle tissue subject to local heating and then to predict the enhancement in thermal conductivity that results from the observed changes in vascular diameter and flow. In contrast to existing studies, which have tried to relate changes in tissue thermal conductivity to local blood perfusion using thermal clearance and self-heated thermistor techniques, we have developed a two-dimensional muscle tissue preparation in which the hyperemic response has been quantified by measuring the in vivo changes in diameter and blood flow of 1A to 4A generation vessels of rat cremaster muscle when the temperature was raised in 2 degrees increments from 34 to 42 degrees C. Only 3A and 4A vessels showed vasodilation when subject to hyperthermia, indicating that the measured increase in flow in the 1A and 2A vessels was the result of a decrease in downstream resistance. Our cremaster muscle preparations have also been used to obtain the first detailed anatomic measurements of the number density and length of countercurrent vessel pairs between 50-200 microns diameter. These combined measurements have been used to establish the limits of validity of the Weinbaum-Jiji theory. Our experimental data indicate that the Weinbaum-Jiji expression for keff is valid in cremaster muscle and cat mesentery tissue for both normal and hyperthermic conditions provided the largest vessels are < 200 microns in diameter. The theory predicts that significant enhancements in keff start to occur for vessels that are 70 microns in diameter or larger, that a 2.5-fold increase in keff can be achieved for a maximally dilated 200 microns diameter 1A vessel pair in cremaster muscle of larger rats, and a 6-fold increase is predicted for maximally dilated 200 microns diameter vessels in the cat mesentery. The experiments also show that maximally dilated 1A to 4A vessels in the microcirculation closely satisfy the condition Q(flow)/(2a)3 = constant, which is consistent with the hypothesis that there is an adaptive regulation of vessel diameter which keeps the wall shear stress nearly constant during temporal changes in flow.

Tumor Blood Flow Measured Using Dynamic Computed Tomography

A method for measuring the tumor blood flow (TBF) by means of dynamic computed tomography (DCT) was devised and investigated. Twenty-seven patients with superficial tumors underwent measurement of TBF by the new DCT method. For all of the 27 patients, the thermal clearance method was employed to measure the relative tumor blood flow (RTBF). For 16 patients, TBF was measured by oxygen-15-gas positron-emission tomography (PET). The TBF values measured by DCT were compared with the RTBF by the thermal clearance method and the TBF by PET. The results demonstrated a linear relationship. The newly devised DCT method yields reliable data in measuring TBF.

A thermal clearance probe for continuous monitoring of cerebral blood flow.

Cortical thermal clearance provides a practical means of continuous assessment of cerebral blood flow. A probe which can be used to monitor cerebral cortical thermal clearance is described. It has proved a reliable method of predicting oligaemia, showing good correlation between measured thermal clearance and the onset of clinical signs of ischaemia. The limitations are its size, invasiveness, small sample volume and difficulty with quantification. It is a useful additional tool in the management of patients suffering from acute neurosurgical disorders.

Increased role of skeletal muscle in the calorigenic response to glucagon of cold-acclimated ducklings

The site of the calorigenesis observed in birds after glucagon was sought in control and cold-acclimated (CA) ducklings. Twenty-four 6-wk-old muscovy ducklings were reared either at thermoneutrality (TN) (25 degrees C) or in the cold (4 degrees C) from the age of 1 wk. Glucagon-induced calorigenesis (GIC) was estimated at 25 degrees C after a peritoneal glucagon injection (103 nmol/kg). Glucagon induced a higher increase in animal heat production (indirect calorimetry) and body temperature in CA (+45% and +1.1 degrees C) than in control ducklings (+30% and +0.4 degree C). In CA ducklings, the perfusion rate (thermal clearance method) and temperature of gastrocnemius increased (+130% and +1.0 degree C) shortly after glucagon, whereas tissue oxygenation (polarography) decreased (-34%). There was no significant effect of glucagon in TN controls. These changes, which peaked 45-60 min after glucagon injection, indicated a prolonged increase of muscle O2 consumption in CA ducklings. Leg muscle blood flow (radioactive microspheres) measured 45 min after glucagon was slightly increased in controls (+20%; P < 0.05), while in CA ducklings, the increase was larger (+76%; P < 0.05). The arteriovenous difference in O2 content was not markedly affected by glucagon in both groups. These parameters indicated an increase in leg muscle O2 uptake in response to glucagon of +29% in controls and +76% in CA ducklings. In controls, 28% of the GIC measured in vivo could be attributed to whole body skeletal muscles, compared with 53% in CA ducklings. The remaining part might be accounted for mostly by the liver.(ABSTRACT TRUNCATED AT 250 WORDS)

Faster procedure for deriving regional blood flow by the noninvasive transient thermal clearance method.

The noninvasive transient clearance method provides absolute quantitative measurement of skin blood flow. It is based on thermally insulating the skin under investigation and measuring the time constant of the resultant exponential skin temperature increase. Conventional assessment of the time constant, by measuring the temperature increase until final equilibrium skin temperature is achieved, has the disadvantage of long time of measurement. A procedure shortening the measurement time is presented. The time constant of the exponential temperature increase is calculated without obtaining the final equilibrium temperature. The method can be used for calculating the time constant of an exponential change of any variable, even though the asymptotic value of that variable is not known.