Change In Wall Temperature Research Articles

Experimental Study on Unsteady Characteristics of Shock and Turbulent Boundary Layer Interactions

In this paper, the experimental study on the shock wave and turbulent boundary layer interactions was performed in the Mach 3.4 supersonic low-noise wind tunnel. The angle of the shock generator was θ = 15°, and the unit Reynolds number of 6.30 × 106 m–1. The wall temperature and pressure distribution during the disturbance process of the shock wave and the turbulent boundary layer were obtained based on the temperature-sensitive paints technique and the flush air data sensing system, and the basic flow field of the interactions region was partitioned. Meanwhile, based on the nano-tracer planar laser scattering technique, and the instantaneous fine structures in the interaction region were obtained and the spatiotemporal evolution characteristics of the flow structure were analyzed. The flow visualization images showed that the oscillation position of the induced shock wave satisfied the normal distribution. Compared with the flow visualization images and the temperature results, the correlation between the flow structure of the interactions region and the wall temperature change was obtained. At the same time, the wall fluctuation pressure of the center surface of the shock wave and turbulent boundary layer interactions region was measured by the high-frequency pulsating pressure sensor. The power spectrum density results showed that under the action of the shock wave incident by the shock generator, there were two characteristics frequency signal of 12 and 30 kHz in the induced shock oscillation interval. For the signal of 12 kHz, the frequency value and the amplitude were increased from the turbulent boundary layer to the separation bubble, and the oscillation energy of the induced shock wave was enhanced. The amplitude of the peak signal of each measurement point gradually decreased from the separation bubble to the reattachment zone, and the energy was gradually attenuated. For the high frequency signal of 30 kHz, the frequency variation of each channel was relatively small, relatively stable, and the energy was concentrated.

Viability of basic heterogeneous nucleation studies with thermally diffusive condensation particle counters

HypothesisWhile the lack of efficient tools yielding controllable uniform supersaturations (S) has delayed basic experimental heterogeneous nucleation studies, common diffusive condensation particle counters (DCPCs) would fill this gap if their present substantial S-variation could be minimized. AnalysisFor an initially saturated vapor in two-dimensional (2D) parabolic flow, with discontinuous wall temperature change from Ts to Tc, we calculate the spatial S(x,y) distribution, including the curve Smax(Ψ) of maximal supersaturations versus streamline Ψ. Activation probability curves P(Ts,Tc) are also calculated assuming that nucleation goes from zero to 100% at a critical supersaturation S*. FindingsTwo new approaches to achieve a nearly constant Smax(Ψ) are discovered. (i) Sampling only the central 50% of the flow is most effective because the [dSmax(Ψ)/Dψ]Ψ=0 = 0. This advantage is lost in the more common axisymmetric configuration. (ii) When the ratio Le = α/D between gas–vapor heat and mass diffusivities is unity, we find the quite general property that Smax(Ψ) is exactly constant. This singular condition may be achieved in special vapor/gas mixtures (ethanol/CO2; methanol/CO2; H2O/air, all seeded with lighter or heavier gases). With greater generality, Le = 1 also in turbulent flows. Therefore, basic heterogeneous nucleation studies with newly available seed particles of fixed size and composition are viable in DCPCs.

Wall Temperature Change and Behavior of Fuel Film after Fuel Spray Impingement on Aluminum and Quarts Glass Wall

Wall Temperature Change and Behavior of Fuel Film after Fuel Spray Impingement on Aluminum and Quarts Glass Wall

Approximate model for evaluation of thermal-hydraulic transients associated with rapid power increase in research nuclear reactor

A simple model, for the estimation of changes in the nuclear fuel element cladding temperature as well as the conditions of the formation of the onset of nucleate boiling, is proposed. The results of this estimation are sufficient to assess the effect of the transient with the peak of the reactor's power on the device's safety, without the need for time-consuming thermal calculations. The basic parameters determined using the proposed model are the maximum wall temperature of the device in a hot spot, the time constant of the wall temperature change, and the course of changes in the onset of nucleate boiling ratio in time. The model may be used for investigating the thermal behavior of thin heat-generating and water-cooled elements (such as fuel elements or uranium irradiation targets) during rapid power rise. The results of the temperature estimation with the proposed model were tested considering the hot spot in the MR-6 type nuclear fuel. The SN code with coupled kinetics and thermal-hydraulics, developed in the MARIA reactor was used to validate the results. The maximum cladding temperature, the thermal time constant and the onset of nucleate boiling ratio parameter simulated by the SN code and the proposed scheme appeared to be very consistent.

Study on Dynamic Heat Transfer Process of Waste Gas in Coke Oven Riser Coking Cycle

Waste heat recovery of waste gas in rising tube is a complex heat transfer process, and this paper analyses and studies the heat transfer problem of the coke hoist heat exchanger in 1 coking cycle by theoretical derivation and numerical calculation. Based on the thermal balance equation, the heat transfer differential equation of the gas flow of the rising tube is derived, the mathematical model of heat transfer for waste gas waste heat recovery is established, and the heat transfer and heat transfer coefficient of the coke oven rising tube heat exchanger in 1 coking cycle are obtained. In 1 coking cycle, the heat transfer of waste gas fluctuates in 1 coking cycle, and the heat transfer of waste gas in the rising tube is 19 to 26 kW, with an average heat transfer coefficient of 30 to 36 W/(m2·K). In the late coking, with the decrease of waste gas flow and the decrease of heat transfer, it will cause the temperature of the rising tube wall to change, and the gas flow should be maintained in the late stage of coking to prevent the phenomenon of sudden change of the inner wall temperature; In the actual production can be several coke ovens in parallel production, and the flow and temperature of waste gas timely monitoring, so that the flow of waste gas control at 800m3/h, improve the efficiency of waste gas waste heat recovery.

Can changes in hoof wall temperature and digital pulse pressure be used to predict laminitis onset?

PICO question
 In horses and ponies at risk of laminitis, does the use of hoof wall temperature and digital pulse pressure as diagnostic techniques for acute laminitis provide a method of detecting acute laminitis in the prodromal stage?
 Clinical bottom line
 
 A palpable bilateral increase in forelimb hoof temperature maintained for longer than half a day may indicate that the horse is 18–­24 hours from acute laminitis onset.
 A period of increased digital pulse may also be expected up to 11 hours prior to onset.
 Further studies using larger and more representative cohorts are required to confirm the accuracy of the times at which such changes can be expected.
 
 

Analysis of Combined Natural Convection and Radiation Heat Transfer in a Partitioned Rectangular Enclosure with Semitransparent Walls

In this paper, we present a two-dimensional numerical analysis of the conjugate natural convection and radiation heat transfer in a double-space enclosure with two semitransparent walls. Two kinds of boundary conditions are considered, the first being the isothermal process of the opaque wall, and the other the incidence of a constant radiation flux in the left semitransparent wall. The renormalization group k–ε model is adopted to simulate the turbulent flow in the enclosure. To compute the radiation heat transfer in a semitransparent medium, the discrete ordinates model is used. We compare the behaviors of enclosures with single and double semitransparent walls and determine the difference in the results obtained for semitransparent and opaque partitions. The results indicate that a semitransparent partition facilitates a reduction in the heat loss or obtains a higher temperature distribution. The transmittance of a semitransparent wall has a great effect on the thermal and flow characteristics in an enclosure. The change of wall temperature is found to be significant when the thermal conductivity values range from 0.05 to 0.5 W/(m·K), and to be small when ranging from 0.5 to 10 W/(m·K). These conclusions are helpful for green design and energy saving in solar buildings.

Purge flow effects on rotor hub endwall heat transfer with extended endwall contouring into the disk cavity

Efficiency improvements for gas turbines are strongly coupled with increasing turbine inlet temperatures. This imposes new challenges for designers for efficient and adequate cooling of turbine components. Modern gas turbines inject bleed air from the compressor into the stator/rotor rim seal cavity to prevent hot gas ingestion from the main flow, while cooling the rotor disk. The purge flow interacts with the main flow field and static pressure field imposed by the turbine blades. This complex interaction causes non-uniform and jet-like penetration of the purge flow into the main flow field, hence affecting the endwall heat transfer on the rotor. To improve the understanding of purge flow effects on rotor hub endwall heat transfer, an unshrouded, high-pressure representative turbine design with 3D blading and extended endwall contouring of the rotor into the cavity seal was tested. The measurements were conducted in the 1.5-stage axial turbine facility LISA at ETH Zurich, where a state-of-the-art measurement setup with a high-speed infrared camera and thermally managed rotor insert was used to perform high-resolution heat transfer measurements on the rotor. Three different purge flow rates were investigated with regard to hub endwall heat transfer. Additionally, steady-state computational fluid dynamics simulations were performed to complement the experiments. It was found that the local heat transfer rate changes up to ±20% depending on the purge flow rate. The main part of the purged air is ejected at the endwall trough location and swept towards the rotor suction side, which is caused by the interaction of main flow and the cavity extended endwall design. The presence of low momentum purge flow locally reduces the heat transfer rate. Changes in adiabatic wall temperature and heat transfer (depending on purge rate) are observed from the platform start up to the cross passage migration of the secondary flow structures.

Natural convection from perforated vertical fins with different hole diameters

Natural convection is a physical mechanism that is mostly benefited in cooling of electronics. Due to the variety of the geometrical and operational parameters, industrial and scientific studies are continuing for better performance. One of the focuses is on perforations of heat sink fins as a passive flow control technique. This work experimentally investigates a sinusoidal wavy fin heat sink after fins were perforated with two different hole diameters. Heat sink was heated by using an electrical resistance for six different heating powers. The temperature at the heater-heat sink interface was measured with the aid of four thermocouples. Transient and steady temperature values were measured and then recorded by means of a data-logger. The details of the experimental setup are given alongside of visuals. It is desired to state some assessment and evaluations about the experimental setup. Related literature studies are also summarized in the introduction section. Heat transfer, Rayleigh and Nusselt numbers were calculated and compared with each other and parameters by means of 2D plot graphics. The time for reaching steady state is changing between 1.5 to 2 hours. Average wall temperature changes linearly with heating power. Average wall temperature values are between 300 and 350 Kelvin. Nusselt number decreases with increasing Rayleigh number. While Rayleigh number changes between 500,000 to 2,000,000, Nusselt number decreases from 425 to 250. It is concluded that increasing perforation hole diameter slightly increases convection heat transfer. Some remarks for the future work is given in conclusion section.An Erratum to this article was published on 31 December 2020 https://dergipark.org.tr/en/pub/ijeat/issue/57873/850296

Two-phase cooling using HFE-7100 for polymer electrolyte membrane fuel cell application

The aim of this paper is to investigate two-phase heat transfer of HFE-7100 in mini-channels and its cooling performance in order to determine its effectiveness for a fuel cell application. Firstly, characteristics of two-phase boiling heat transfer of HFE-7100 is analyzed. It is demonstrated that two-phase boiling heat transfer coefficient of HFE-7100 in mini-channels is strongly dependent on heat flux and vapor quality but less sensitive to mass flux. Critical heat flux is observed when wall superheat is over 25 K and flow visualization method is used to examine a flow pattern change.In the second part, cooling performance and wall temperature change is investigated. Under heat generation ranges from a fuel cell, wall temperature is maintained at desirable operating temperature of polymer electrolyte membrane fuel cell (60–80 °C) and temperature difference is lower than 0.5 °C. At critical heat flux condition, wall temperature rises to over 90 °C. Also, wall temperature increases from 63.5 °C to 71.6 °C when channel pressure rises from 1.0 bar to 1.5 bar. Lastly dynamic response in wall temperature with the coolant pump control is investigated. It is found that this control method is effective to maintain low temperature difference (less than 1.5 °C) and the intermittent pump operating mode can reduce pump energy consumption.

Dynamic Heat Transfer Study of a Triangular-Shaped Latent Heat Storage Unit for the Attic Space of a Domestic Dwelling

Abstract A two-dimensional (2D) numerical study is carried out to investigate the thermal performance of an impure phase-change material (PCM) in an equilateral triangular-shaped double pipe heat exchanger. To tackle the irregular boundaries, a nonorthogonal body-fitted coordinate (BFC) transformation technique is employed. The nondimensional transformed curvilinear conservation equations for mass, momentum, and energy are written in terms of physical variables and they are solved using a control-volume based finite difference method on a staggered grid arrangement. The developed model is then used to study the effects of the inner tube wall temperature, the initial temperature of the solid PCM, and the shape, as well as the position of the inner tube in the annulus on the melting characteristics, and cumulatively stored energy. Various quantities such as average Nusselt numbers over the inner tube surface, the total and complete melt fractions, and the latent and total stored energies all as a function of the melting time are reported. A correlation for the average Nusselt number on the inner tube wall is also provided. The numerical results show that the shape and the placement of the inner tube are crucial for the efficient design of a latent heat thermal energy storage (LHTES) system. The storage of energy is greatly influenced by the change of the inner tube wall temperature compared to the change of initial solid PCM temperature.

Non-uniform wall temperature distribution of nucleate boiling heat transfer in helically coiled tubes

Numerical simulation is carried out for nucleate boiling heat transfer of a helically coiled tube. The boiling heat transfer characteristics and the non-uniform wall temperature distribution are in basically consistent with existing experimental data. The influences of operating conditions and shapes on wall temperature are discussed in detail. The results show that the liquid gathers at the outside and the bottom of the cross section during nucleate boiling heat transfer, resulting in a lower wall temperature in these locations. However, the movement of the vapor phase to the top and the inside walls results in higher local wall temperatures. The maximum temperature difference throughout the cross section is approximately 10 K. Furthermore, the distribution of vapor volume fraction becomes uniform as the secondary flow enhances. Due to the vapor–liquid phase distribution, the wall temperature of the cross section shows a sharp rise before decreasing. Under different experimental conditions, the position of the maximum wall temperature changes from 90° to 180° and the maximum deviation is more than 60°.

Conductive Heating of a Confined Gas

This paper addresses the heating of a gas confined in a closed vessel whose wall temperature changes with a time scale much longer than the vessel acoustic time, so that the relative spatial pressu...

Simulation to study the effect of natural convection in the voids of a 2D granular packed bed of cylindrical particles on effective thermal conductivity

Natural convection can occur in the voids of a granular material due to temperature difference between neighboring particles. Such an effect of natural convection in voids of a granular material can have a significant impact on effective thermal conductivity of that granular material. In the present study, effective thermal conductivity of a packed bed of solid cylindrical steel rods filled with air and water in the voids is analyzed by simulation. Effective thermal conductivity is obtained for the two systems from a response to step change in wall temperature. In order to quantify the effect of natural convection, a two dimensional array of squarely packed cylinders is simulated with and without the natural convection effect. The simulation is carried out using COMSOL Multiphysics. The temperature versus time data for various particle sizes of 4mm, 8mm and 12mm made of steel are obtained. The maximum velocity magnitude generated in the voids is recorded as a function of time as a measure of natural convection. The effective thermal conductivity is determined for all the conditions using curve fitting between simulation data and analytical function. The effective thermal conductivity calculated from simulation data for two conditions namely with and without natural convection are compared with the literature correlations for both the systems. Role of Grashof number on the intensity of natural convection in voids is discussed. Aspect ratio effect of the packed bed and mesh size effect are also studied.

The Effect of Inlet Velocity on CH4 Catalytic Combustion Behavior with H2 Addition in a Microchannel Combustor

AbstractThe characteristics of premixed methane/air combustion in a microchannel with a Pt catalyst were examined and the effect of inlet velocity on catalytic methane combustion with hydrogen addition was investigated numerically. It is shown that appropriate hydrogen addition can expand the extinction limits, particularly the upper extinction limit of the inlet velocity, and increase methane conversion at higher inlet velocities. However, it is proved that methane conversion at an equivalent ratio of 0.8 decreased with hydrogen addition when inlet velocity was lower than 0.36 m s−1. The heat released in the hydrogen–oxygen reaction increased the wall temperature, which resulted in the adsorption–desorption equilibrium of oxygen shifting towards desorption as Pt(s) coverage increased and O(s) coverage decreased. With the same amount of hydrogen added, the changes in wall temperature that reflected the thermal effects of hydrogen were more obvious at higher inlet velocities, which improved methane conversion. If the heat released by hydrogen was neglected, the competitive consumption of oxygen, which demonstrates the chemical effect of hydrogen, was more significant at lower inlet velocities, which inhibited methane conversion.

Experimental Study on Dynamic Thermal Environment of Capillary Radiant Floor Heating Room with Finite Heat Source

The heat storage tank has an important role in regulating temporal distribution of solar energy, but its heating characteristics are significantly different from those of the traditional constant heat source system due to the limited heat storage capacity. In order to explore how to effectively utilize the solar energy to improve the indoor thermal environment, a solar heating system (solar flat panel collectors + a heat storage tank + capillary radiation floor) was set up. The dynamic changes regularity of supply water temperature, return water temperature, indoor air temperature and enclosure inner surface temperature were tested during the heating process. The formation mechanism of the dynamic thermal environment was revealed through the theoretical analysis of the comprehensive heating capacity and heat storage capacity of capillary radiation floor. The results show that initial exothermic power of heat storage tank is large but declines rapidly and a ‘cliff’ temperature reduction of supply water leads to the changes of indoor temperature and wall temperature, which can be divided into three stages, rising, stability and falling. The formation mechanism of the dynamic thermal environment lies in the unique manifestation of the comprehensive heating capacity and the percentage of net radiant heating capacity of floor surface.

Turbulent boundary layer response to the introduction of stable stratification

The response of an initially neutral rough-wall turbulent boundary layer to a change in wall temperature is investigated experimentally. The change causes a localized peak in stable stratification that diffuses and moves away from the wall with downstream distance. The streamwise and wall-normal components of turbulent velocity fluctuations are damped at similar rates, even though the stratification only directly impacts the wall-normal component. The Reynolds shear profiles reveal the growth of an internal layer that scales approximately with the bulk Brunt–Väisälä frequency.

The experiment study of the sensing mechanism of thermal magnetic type oxygen sensor

The oxygen sensor is widely used in daily life. Thermal magnetic type oxygen sensor has some advantages such as long service life and high accuracy of measurement. It skillfully uses the change of the wall temperature that caused by the thermal magnetic convection phenomenon to sense the change of oxygen concentration. For a paramagnetic gas, the thermal megnetic convection is caused by the interaction between the temperature field and the magnetic field as the gradient of the magnet field and the temperature gradient both existing at the same time. The intensity of thermal megnetic convection is determined by the magnetic susceptibility of a gas and the gradient of H 2 (where H is the induced magnet field). Oxygen is a common paramagnetic gas, the magnetic susceptibility of oxygen has a much higher than other gas, thus, the oxygen concentration of a gas can be detected by the thermal magnetic convection phenomenon. However, the thermal magnetic convection phenomenon is complicated, The sensing mechanism of this type sensor is not clear. Therefore, it is necessary to study the sensing mechanism in-depth to improve the accuracy and efficencey of thermal magnetic type oxygen sensor. This paper, by means of experiments, studies the wall temperature change characteristics of the thin-wall circular tube in a non-uniform magnetic field generated by two pieces of rectangular permanent magnets. In the experiment, a quartz glass tube with a thin wall thickness was used as the channel through which the measured gas flows. Two permanent magnets are arranged outside the quartz tube, which are used to provide the magnetic field required for the thermal magnetic convection. A kind of the sensitive element with double helically wounded resistance wires is designed. In which two resistance wires with different temperature coefficient are helically wounded on the wall of outside the quartz tube, one is the Manganin resistance wire for heating, and one is the platinum wire for measuring the temperature of tube wall. The loops of the heating and the temperature measurement are separated, which is favorable for study of the thermal magnetic type oxygen sensor. The permanent magnets used in the experiment have the remanence of 1.32 T. The ambient temperature around the sensitive element is measured by the thermocouple. The experimental device, which includes the sensitive element and some appropriately insulating elements, is placed in a rectangular cavity, and the size of the square cavity is much larger than the sensitive element. By comparing temperature difference of the tube wall, the increment of the convective heat transfer coefficient and the increment of velocity under conditions with and without the magnetic field to illustrate the relationship of the thermal magnetic convection phenomenon and heating power, oxygen concentration and other parameters. It is found that when magnetic field is pesent, the surface temperature of the tube wall is significantly lower than that of the case without magnetic field, and with the increase of heat flux, the surface temperature of tube wall decreases significantly. The relationship between the heat flux density and the change of the wall temperature is nearly liner.With the increase of heat flux density, the increasments of average velocity and convective heat transfer coefficient caused by the thermal magnetic convection phenomenon increase. When oxygen content increases by 10%, the temperature of the tube wall decreases by 0.12°C. The resolution of the studied oxygen concentration senser is about 0.08%.

Prediction of dryout and post-dryout wall temperature at different operating parameters for once-through steam generators

A three-dimensional “unit tube” physical model is established by simplifying the once-through steam generator (OTSG) designed by B&W and a three-dimensional two-fluid three-flow-field mathematical model that takes into account the interactions between liquid droplets and other flow-fields (steam and liquid film) and the interactions between liquid droplets and the wall in liquid deficient regions is introduced. The results of a numerical simulation using the developed OSTG model conducted at different operating parameters show that the three-dimensional two-fluid three-flow-field mathematical model used in this paper can accurately predict the dryout and post-dryout wall temperature distribution of the OTSG at different operating conditions. The simulations also show that the post-dryout wall temperature decreases along the height of the heat transfer tube at low mass fluxes while gradually increases at high mass fluxes, which is opposite to the changes in wall temperature seen in post-departure from the nucleate boiling region. Furthermore, dryout occurs later as pressure increases, with a smaller soaring maximum of the wall temperature. Dryout will occur when the wall temperature gradient reaches or exceeds 250K/m as the heat flux increases.

A new approach and results of wall and air temperature dynamic analysis in underground spaces

In this paper our primary aim is to define the changes of air and internal wall temperature in underground spaces in time domain. As an additional aim the change of heat flux through the wall in time domain has been calculated. Based on the heat balance, the dynamic basic equation of the space has been defined. The basic equation is a differential equation which contains the internal heat sources and the heat capacity of the space. For solving the basic equation, the initial condition, the time-varying boundary condition of the third kind and the Fourier's conductivity differential equation are necessary. The convolution integral of the solution function has been obtained by the use of the integral-differential equation acquired by substituting the temperatures and heat fluxes into the basic equation. The solution of the acquired equation can be obtained in a numerical way. Our new mathematical approach to the solution of the physical model makes it possible to investigate the air and wall temperatures, as well as the heat flow through the wall in underground spaces.