Calculated Temperature Difference Research Articles

CALCULATION OF NORMAL TEMPERATURE DIFFERENCE ON MAIN GAS PIPELINES WITH LONG SERVICE LIFE

In the global pipeline transport, including main pipelines (MР) with a long service life (40 years or more), the reliability of operation is ensured by compliance with regulatory requirements. Calculations of the stress-strain state of gas pipelines, in particular, longitudinal stresses, are carried out in accordance with Standart SP 36.13330.2012 «Main pipelines», which does not take into account the life of the facilities.
 The analysis of SP 36.13330.2012 «Main pipelines» for the effect of the temperature factor on the magnitude of the longitudinal stresses of the gas pipeline section showed that the calculation of longitudinal stresses from the temperature difference ∆Т provides for taking into account four values: Тс – maximum and minimum temperatures during construction; Tр – the highest and lowest temperatures during operation.
 When considering the stages of construction of gas pipelines, it was established that during this period the pipeline experiences the action of four temperature regimes, and when put into operation, the fifth complex one: from the temperature and pressure of the gas, at which the consolidation of the main pipeline with the surrounding soil occurs predominantly in during which the linear-altitude position of the MР section with a minimum of stresses is predominantly formed.
 It has been proved that, in relation to the calculations of longitudinal stresses of gas pipelines with a long service life, it is necessary and sufficient to limit the calculated temperature difference by Tр.
 SP 36.13330.2012 is intended for calculations during the construction of new pipelines, and its use for long-term operated gas pipelines, taking into account the standard value of ∆Т, mainly leads to rejection, and, therefore, to repair.
 We believe that for the operation of MР with a long service life, for the calculation of thermal stresses, it is necessary to develop a standard that allows the calculation of the temperature difference ∆Т by the value of the operating temperature Tр.

SELECTING THE HEAT-REFLECTING SCREEN WHILE THE FORMATION OF CONDENSATE IS AVOIDED

This article deals with the problem of choosing the thickness of the heat-refl ecting screen behind the heating radiator, in which the conditions for the formation of condensate on the wall are not achieved. Based on the analysis of the results obtained, the maximum thickness of the screen is established, using the example of penofol, at the calculated temperature diff erence in the conditions of a cold fi ve-day period, and the heat losses and the economic eff ect of this energy-saving measure are determined already during the heating period. Calculations were carried out for a wall made of reinforced concrete, covered with plaster on the outside. This technique can be used for other cases of enclosing structures and various materials of the heat-refl ecting screen.

The Efficiency of Vapor Compression Transformation of Energy Flows for Heat Supply Based on the Sea Water

The results of the analytical study substantiated the operating conditions for the highly efficient use of the temperature potential of seawater in heat pump heating systems (HPHS) for buildings a building with correspondingly improved environmental indicators. Based on the analysis of the regional conditions of the Odessa water area of the Black Sea, the initial parameters have been substantiated and rational modes of operation of an improved HPHS with central, decentralized or local heating of the subscriber energy carrier have been determined. As indicators for evaluating the efficiency of the HPHS operation, the conversion factor of energy flows and the specific consumption of external energy for the drive of the compressor and the circulating pump of cooled water in the operation of heat pump units were considered. For seawater in the Odessa water area of the Black Sea during the entire heating period, the following temperatures were considered as initial data for analysis: water at the inlet to the evaporator (5–10) ° C, at the outlet (1 °C); calculated temperature difference of the coolant in the heating system (50–40) °C, indoor air (20 °C); estimated outdoor temperature (–18 °C). The characteristic correspondence between the flow rates of the cooled sea water and the heated energy carrier of the heat supply system was taken into account. The prerequisites of high efficiency of the heat pump heat supply system in which the actual conversion coefficient exceeds the seasonal normalized calculated and minimum value at an outdoor temperature of (–10) °C under the limiting conditions of the monoenergy regime for both new and reconstructed buildings were substantiated. In the course of the study, it has been determined that the total specific consumption of external energy for the compressor drive and the circulation of cooled water in the operation of a heat pump unit with a characteristic ratio of water equivalents, even under the limiting conditions of the monoenergetic mode of operation of the heat supply system at an outdoor temperature of (–10) °C, are within the range of generally accepted values (w = 0.28–0.34).

3 GHz RF measurements of AlGaN/GaN transistors transferred from silicon substrates onto single crystalline diamond

The integration of AlGaN/GaN thin film transistors onto diamond substrates enables the efficient dissipation of device heat, thus providing a boost in performance and reliability of current high-frequency GaN power amplifiers. In this paper, we show 3 GHz load-pull measurements of GaN transistors on silicon (Si) and single crystalline diamond (SCD) as fabricated by our recently presented direct low-temperature bond process. After the transfer onto SCD, the efficiency and output power are increased by 15%, which is explained by a calculated temperature difference of ∼100 K. In addition, the temperature between individual gate fingers is reduced such that the output power density (Pout) is independent of the amount of fingers. A drawback of our GaN epilayer is identified in the huge thermal resistance of the buffer layer so that the heat spreading performance of our technology is significantly impaired. Nevertheless, we demonstrate a large GaN-on-diamond output power of 14.4 W at a Pout of 8.0 W/mm.

EXPERIMENTAL DETERMINATION OF CONVECTIVE HEAT TRANSFER COEFFICIENTS IN THERMAL BUOYACY VENTILATION SYSTEM

Objectives. The main goal of the article is to present the developed method for the experimental determination of convective heat transfer coefficients, suitable for studying the internal convection of models of complex configuration. Method. The study of free convection under the conditions of an internal problem was carried out by determining the conditional thickness of the boundary layer by a graphic method. The first was the selection of the calculated sections and planes for the experimental installation. The selection is carried out in such a way that the calculated planes are perpendicular to the heated walls of the channel in question. Installation of an experimental model is possible only in a room with low internal air mobility, as well as a stable temperature. In this room there should not be heating and heating devices that can create strong convective currents near the channel of the experimental installation. Result. The article presents the results of an experimental study to determine the temperature distribution of the air flow and average convective heat transfer coefficients over the height of the ventilation channel. A decrease in convective heat transfer coefficients at an altitude of 0.5 to 1 meter occurs less noticeably than at an altitude of 1 to 2 meters, which is associated with the restoration of flow after a vent removal. At the stabilization section, there is first a gradual decrease, and then an increase in axial velocity, which is caused by the merging of multidirectional air flows in this area. Conclusion. It was revealed that in the case of modeling free convection under the conditions of an internal problem in the presence of heat-removing boundaries within the limits of the calculated temperature difference, taking into account the flow turbulization has practically no effect on the final results.

SIMULATION OF FREE CURRENT FLOWS IN BUOYANCY-DRIVEN VENTILATION SYSTEMS

Objectives . The aim of the study is to analyse the effect of the design and methods for heating the ventilation duct of a buoyancy- driven system on the formation of free convective air currents in it. Methods. The study of free convection under the conditions of interior problem was carried out using the CFD software, based on the finite volume method with unstructured grid. Ansys Fluent software was used as a calculation tool in the study, due to its having a high convergence of numerical solutions offering full-scale measurements of convective currents.To evaluate the reliability of the results obtained, a validation procedure was carried out, allowing us to determine how accurately the selected conceptual model describes the investigated flow through a comparison of experimental and numerical data. Results . The results of numerical modelling of free convective currents occurring in the heated channel of the ventilation system of the top floor of a multi-storey residential building are presented in the article. In the course of the study, the air velocity at the entrance to the ventilation duct was found to depend on the calculated temperature difference θ ˚C for various heating methods. A gradual increase in the discrepancy between the numerical simulation and experimental results is observed if the calculated temperature difference > 20 ° C. This phenomenon is due to the fact that with increased duct temperature, it is quite difficult to achieve even heating under actual conditions; this is especially noticeable when considering the variant when the vertical part of the vent duct and the take-off are both heated. The maximum deviation of the results is 4.4%. The obtained velocity profiles in the calculated sections indicate the impact of the ventilation take-off on the nature of the air flow motion. Conclusion . One of the drawbacks of the existing systems of natural ventilation of residential buildings is the low efficiency of work in the warm and transitional periods of the year, especially in bathrooms. The use of buoyancy-driven ventilation with vertical heating of the vent duct combined with the take-off allows a stable air exchange in the rooms to be provided.

Monitoring tumor state from thermal images in animal and human models.

Thermography is a potentially useful method for tumor progress monitoring since it is noninvasive, nonradiative, low-cost, and rapid. Perfusion and metabolism are dominant factors for determining tumor temperature difference and are also correlated to the tumor's growth rate. Therefore, estimating them from the tumor thermal image can be a very useful tumor monitoring method, since thermal changes occur before physical changes. The goal of this work was to study the effect of tumor state on the thermal image in different tumor types, using simulations and measurements. Simulated tumor models, representing flat and extruding tumors, typical to transplantable and natural tumors, respectively, were simulated and the effects of tumor metabolism and perfusion on the temperature difference were analyzed. Data regarding tumor size and measured temperature differences were obtained from the literature, discussing five types of transplantable tumors in mice and rats. The growth rates of all tumors were calculated by fitting tumor size measurements to a tumor growth model and were used as an indicator to tumor aggressiveness. Tumor temperature difference was calculated by taking the effect of its extruding shape into account, according to a previously published method. Tumor state was estimated from the normalized temperature differences using simulations and compared to the calculated aggressiveness rates. Computational models of human breast cancers, both in round and flat breast models, were recreated using a finite-element-method heat transfer simulation. Tumor size and state were simulated according to the results obtained from the animal tumor analysis, representing two different tumor aggressiveness levels. The calculated temperature difference as a function of tumor size was calculated for each test case. Perfusion was shown to be highly dominant in determining the tumor's temperature difference. Since both metabolism and perfusion were shown to have a linear effect on the temperature difference, a conversion value was defined between them. The analysis of the animal experimental results showed correlations between tumor aggressiveness and the following factors: the normalized temperature difference, the estimated tumor state, and the temperature difference change rate. The simulated human breast cancer models analysis showed highly varying temperature differences between the simulated models. Although for each model there is a clear difference between the temperature differences of the test cases simulated, the large differences between the results might make tumor state estimation difficult. However, reviewing the gradient of the tumor temperature change as a function of tumor size showed that the ratio between the gradients of both test cases was similar for all models. Therefore, the effect of model errors and differences in the simulated tissue structure and properties and the environmental conditions between the different models, can be mitigated. This pattern may be used to estimate tumor state in in vivo experiments. Continuous monitoring of tumor temperature difference produces valuable information on tumor state and aggressiveness that can be used both in the clinic and in the laboratory. Monitoring can be either performed on a single image, or continuous on multiple images, revealing changes in tumor state.

Virtual calibration of a supply air temperature sensor in rooftop air conditioning units

Supply air temperature (SAT) measurement is an important element in sequencing control and automated fault detection and diagnosis (AFDD) in HVAC systems to ensure the comfort of building occupants, decrease energy consumption, and lower maintenance cost. But in rooftop air conditioning units (RTUs) with gas-fired heating, the accuracy and reliability of manufacturer-installed supply air temperature (MSAT) sensors are notoriously difficult to attain. Experimental evaluations in this study, covering both the cooling and heating modes and using both direct measurements of a MSAT sensor and a multi-sensor measuring grid, demonstrate that direct measurements cannot obtain the true value of SAT in RTUs in the heating mode. Erratic measurement errors exist due to nonuniform temperature distribution and intensive thermal radiation in a compact chamber. An innovative indirect virtual calibration method for an MSAT sensor is proposed in this article to solve this issue. It demonstrates that a virtual calibrated MSAT sensor can provide accurate results when combined with a linear correlation for offset error that depends on heating stage and outside air damper signals. The linear correlation could be determined using the calculated temperature difference between the predicted theoretical true value of SAT and the direct MSAT measurement. This virtual calibration method is generic for all RTUs with similar construction of gas furnaces and can be implemented for long-term use. Further experimental evaluation and uncertainty analysis prove that the virtual calibration method can accurately predict the true value of SAT in RTUs within ±1.2°F (0.7°C) uncertainty. This economical technology will not only improve energy management of packaged units in sequencing control but also better facilitate real-time automated control and fault detection and diagnosis.

A Numerical Study on Condensation in a Heat Pipe Using a Binary Mixture.

A numerical analysis has been made on a reflux condensation of binary vapor (R 113+R 11) in a two-phase closed thermosyphon in order to predict its variable conductance behavior and to clarify the detailed mechanism. The flow, and heat-and-mass transfer in the vapor phase and the condensate liquid film were solved simultaneously for the axisymmetric two-dimensional field in a circular pipe. It was shown that, in the case of smaller heat load, non-condensing region is formed in the upper part of the condensing section due to the accumulation of more volatile component of vapor (R 11), and the length of the non-condensing region increases with the decrease of heat load. As the result, the temperature difference between the inlet vapor and the condensing wall is kept nearly constant for varying heat load. The calculated temperature difference and system pressure were compared with experimental data, and reasonable agreements were obtained.

Large‐scale thermal bending fracture of sea ice plates

A hypothesis that large‐scale fracture of sea ice plates in the Arctic could be caused by the release of energy of thermal bending moments due to major temperature changes is advanced and examined. Bending propagation of a through‐the‐thickness crack along the floating plate, with negligible inertial forces, is analyzed, assuming the moment field in the plate near the traveling crack front and the fracture process zone to be in a steady state. The analysis uses the plate‐bending theory, and the second‐order geometric effects of the in‐plane normal forces are taken into account. Quasi‐elastic behavior is assumed, and creep is treated approximately according to the effective modulus method. The calculated temperature difference between the top and bottom of the plate required to produce this kind of fracture is found to be well within the range that actually occurs in the Arctic, but this cannot be regarded as a proof of the hypothesis because of the simplifying assumptions made as well as uncertainties about large‐scale fracture properties of sea ice. Further, it is shown that this type of fracture must exhibit a size effect, such that the critical temperature difference decreases in proportion to (plate thickness)−⅜. This might explain why large fractures often form in an intact thick plate rather than only in a thin plate and along lines of weakness. For the case that the in‐plane forces are significant, it is shown that beyond a certain critical crack length the thermally driven bending fracture (if it exists) must transit to a planar (nonflexural) fracture driven by the release of the energy of the in‐plane forces generated by wind and ocean currents. The effect of creep is to increase the required critical temperature difference, as well as the critical crack length for the aforementioned transition. For thermal bending fracture, the minimum possible spacing of parallel cracks increases with the plate thickness and is independent of the crack length, while after transition to planar fracture it increases in proportion to the crack length. The hypothesis of thermal bending fracture cannot be proven or disproven without new types of experiments and measurements in the Arctic, and their computer modeling.

Heat Transfer during Heat-treatment of Carbon Fibers by Direct Currenting

A polyacrylonitrile-based carbon fiber tow was heat-treated by directly passing electric current through the tow. The relation between the surface temperature of the tow and supplied electric power was analyzed by an energy balance equation. The rediation became the dominant factor for the heat-loss at high temperatures. The apparent heatconductivity of tow increased with increasing temperature. Temperature distribution in the crosssection of tow was analyzed by considering the temperature dependences of heat-conductivity and specific resistivity. The calculated temperature difference between the center and the surface of the tow increased with increasing surface temperature. The temperature distribution was reduced by installing a reflector around the tow. It is expected that the temperature distribution in a tow gives rise to a certain distribution of the structure of carbon fibers. This subject was also studied by analysing the intensity profiles of wide-angle x-ray diffraction. The integral width of 002 reflection for the tow heat-treated by direct currenting was calculated by using the analyzed temperature distribution and the heat-treatment temperature versus layer thickness relation obtained for the carbon fibers heat-treated with a conventional furnace. The estimated values agreed well with the experimental ones.

Growth of single crystals of zinc selenide from the vapour phase

More than one hundred crystals of zinc selenide have been grown from the vapour phase using a technique in which the material is sublimed down a temperature gradient in a sealed tube which is connected to a reservoir containing either zinc or selenium. It is found experimentally that maximum transport occurs when the temperature of the resevoir is adjusted to give a vapour pressure of the element in the reservoir equal to that of the same element above the compound under Pmin conditions. It is demonstrated that in these circumstances the vapour above the subliming charge is very nearly stoichiometric. The composition of the vapour over the growing interface is less stoichiometric and is rich in that element which is contained in the reservoir. Using the ideas developed by Faktor et al. [6–9] it is shown that a calculated temperature difference of the order of 20°C is required to provide the observed transport rate. The experimental temperature difference is also of the order of 20°C. The conclusion is that the growth of zinc selenide is limited by the transport processes which occur in the growth capsule.