Change In Wall Temperature Research Articles

Photothermal Spectrometry in Small Liquid Channels

A novel apparatus for performing photothermal lens spectroscopy is described. The apparatus uses a low-volume cylindrical sample cell, a chopped or pulsed excitation laser, and a continuous probe laser. The full volume of the sample is irradiated with constant, e.g., non-Gaussian, irradiance beam produced by the excitation laser. Constant irradiance excitation source does not directly produce the photothermal lens element in the sample. The lens element is formed by thermal diffusion from the irradiated sample volume, through the sample cell walls. Under continuous irradiation, thermal diffusion results in a parabolic temperature change profile. The apparatus has been found to work with cells designed to contain sample volumes from 6 μL down to 24 nL. Larger and smaller volume cells are practical but signals produced in the smallest sample cells exhibit deviation from that expected based on the theory. This is attributed to probe laser diffraction by the circular aperture of the sample cell. (Received on August 9, 2001, Accepted on......) Introduction Photothermal lens spectrometry is a novel means to measure optical absorption. It is one of several ultra-sensitive photothermal spectroscopy methods applicable to trace analysis. The applicability to trace analysis is due to the high sensitivity and small sample volumes needed for an analysis. There are several physical effects that limit sensitivity and influence the accuracy of measurements obtained using photothermal lens spectrometry. First, the photothermal lens signal is related to the lens formed the sample as a consequence of light absorption and subsequent energy transfer to the matrix. The optical element produced when laser beams are focussed into a homogeneous sample is not a simple lens. The aberrant nature of the photothermal lens results in signal magnitudes that are somewhat smaller than expected and also more difficult relate to sample absorbance. Second, sample irradiation can produce large temperature changes. In theory, the on-axis temperature change approaches infinity at long times for any excitation power when using continuous laser excitation sources. In practice, large temperature changes distort the thermal lens perturbation due to density differences and convection heat transfer or even boiling may occur. The later distorts the signal and ruins the analytical utility. Other factors resulting in inaccurate absorbance determination are related to the photophysics of sample excitation and solvent (matrix) relaxation. Nonlinear optical absorption and bleaching, transient volume changes, and excited state analyte absorbance can occur when using high irradiance laser excitation sources. These effects may generally be avoided only by using low irradiance. This is often accomplished by choosing continuous over pulsed lasers for sample excitation. An apparatus utilizing a two-laser photothermal lens apparatus that is immune to many problems associated with photothermal lens spectrometry and capable measuring optical absorbance in very small samples is described in this work. The apparatus uses a nanoliter-volume sample cell with a visible laser used as the excitation source. The sample cell is constructed from high thermal conductivity metal and has a large surface area and relatively large heat capacity. A simplified version of the theory for describing the photothermal lens developed in a cylindrical sample cell is given. A model for relating signals to sample absorbance is given and experimental evidence is presented. Timedependent photothermal signals are detected The technique exhibits high sensitivity and can be used for ultra-sensitive detection. Analyte excitation is often performed using laser light sources. The resulting photothermal lens is measured using a probe laser. Excitation and probe lasers may be the same. Laser light sources produce a small spot size at a focus. The small spot size allows one to probe extremely small sample volumes. Theory Theoretical time-dependent temperature changes and inverse photothermal lens focal lengths for both standard and cylindrical sample cells are described in the literature. The cylindrical sample cell used here has a large thermal conductivity compared to the solvent. Little temperature change occurs at the sample cell wall because of the high thermal conductivity ratio. The solution to the thermal diffusion equation for a zero temperature change at the cell wall temperature change is

Thermally developing electroosmotic flow of power-law fluids in a parallel plate microchannel

The present investigation considers the thermally developing electroosmotic flow of power-law fluids through a parallel plate microchannel. Both the viscous dissipation and Joule heating effects are taken into account and a step change in wall temperature is considered to represent physically conceivable thermal entrance conditions. Expressions for the dimensionless temperature and Nusselt number in the form of infinite series are presented. In general, the resultant eigenvalue problem is solved numerically; nevertheless, an analytical solution is presented for the regions close to the entrance. A parametric study reveals that increasing amounts of the Peclet number result in higher wall heat fluxes, whereas the opposite is true for the flow behavior index. Furthermore, based on the value of the dimensionless Joule heating parameter, the Nusselt number may be either an increasing or a decreasing function of the axial coordinate or even both of them in the presence of a singularity point. The viscous heating effects are also found to be negligible.

An optimization method with experimental validation for the design of extrusion wire coating dies for a range of different materials and operating conditions

AbstractThe objective of this article is to determine a wire coating‐hanger melt distributor geometry to ensure a homogenous exit velocity distribution that will best accommodate a wide material range and multiple operating conditions (i.e., die wall temperature and flow rate change). The computational approach incorporates finite element (FE) analysis to evaluate the performance of a die design and includes a nonlinear constrained optimization algorithm based on the Kriging interpolation and sequential quadratic programming algorithm to update the die geometry. Two optimization problems are then solved, and the best solution is taken into account to manufacture the optimal distributor. The Taguchi method is used to investigate the effect of the operating conditions, i.e., melt and die wall temperature, flow rate and material change, on the velocity distribution for the optimal die. In the example chosen, the wire coating die geometry is optimized by taking into account the geometrical limitations imposed by the tool geometry. Finally, the FE analysis and optimization results are validated by comparison with the experimental data obtained with the optimal die. The purpose of the experiments described below is to investigate the effect of material change. POLYM. ENG. SCI., 2012. © 2012 Society of Plastics Engineers

Study on energy saving effect of heat-reflective insulation coating on envelopes in the hot summer and cold winter zone

To discuss the energy saving effects of heat-reflective insulation coating on exterior envelope walls in the hot summer and cold winter zone, the experimental apparatus and the actual test rooms are built respectively, the heat flux data of east, south and west walls are analyzed by monitoring the wall temperature changes of two test rooms with and without the heat reflective insulation coating in Hangzhou in summer and winter. The experimental results indicate that the heat reflective insulation coating could reduce the exterior wall surface temperature effectively, and the maximum temperature change is about 8–10°C. By calculating, it was found that the annual air-conditioning electricity saving with heat reflective insulation coating on exterior walls is about 5.8kWh/(m2month), which shows that the energy saving effect of the heat insulation coating on the exterior wall is obvious in Hangzhou.

Numerical simulations of mixing conditions and aerosol dynamics in the CERN CLOUD chamber

Abstract. To study the effect of galactic cosmic rays on aerosols and clouds, the Cosmics Leaving OUtdoor Droplets (CLOUD) project was established. Experiments are carried out at a 26.1 m3 tank at CERN (Switzerland). In the experiments, the effect of ionizing radiation on H2SO4 particle formation and growth is investigated. To evaluate the experimental configuration, the experiment was simulated using a coupled multidimensional computational fluid dynamics (CFD) – particle model. In the model the coupled fields of gas/vapor species, temperature, flow velocity and particle properties were computed to investigate mixing state and mixing times of the CLOUD tank's contents. Simulation results show that a 1-fan configuration, as used in first experiments, may not be sufficient to ensure a homogeneously mixed chamber. To mix the tank properly, two fans and sufficiently high fan speeds are necessary. The 1/e response times for instantaneous changes of wall temperature and saturation ratio were found to be in the order of few minutes. Particle nucleation and growth was also simulated and particle number size distribution properties of the freshly nucleated particles (particle number, mean size, standard deviation of the assumed log-normal distribution) were found to be distributed over the tank's volume similar to the gas species.

Graetz Problem Extended to Mixed Electroosmotically and Pressure Driven Flow

Thermally developing mixed electroosmotically and pressure-driven flow in a parallel plate microchannel with a step change in wall temperature is considered in the framework of an extended Graetz problem. Both Joule heating and viscous dissipation effects are taken into consideration. Expressions for the dimensionless temperature and Nusselt number in the form of infinite series are presented. In general, the associated eigenvalue problem is solved numerically. Nevertheless, an analytical solution is also presented for axial locations close to the entrance. Comparisons aremade between the present results and those obtained by approximating the electroosmotic velocity with the Helmholtz–Smoluchowski velocity. It is revealed that, for a pressure-assisted flow with a sufficiently high magnitude of pressure gradient, the Nusselt number is weakly dependent on the dimensionless Debye–Huckel parameter. Accordingly, the approximate solution is valid for such cases.However, unless a sufficiently large value of the dimensionless Debye–Huckel parameter is considered, the incurred error rapidly increases when the pressure gradient is absent or acts adversely. The approximate solution also fails to predict accurate results near the singularity points that occur in theNusselt numberswhen thewall temperature in the downstream is higher than that of upstream.

Impacts of Fluid Dynamics Simulation in Study of Nasal Airflow Physiology and Pathophysiology in Realistic Human Three-Dimensional Nose Models

During the past decades, numerous computational fluid dynamics (CFD) studies, constructed from CT or MRI images, have simulated human nasal models. As compared to rhinomanometry and acoustic rhinometry, which provide quantitative information only of nasal airflow, resistance, and cross sectional areas, CFD enables additional measurements of airflow passing through the nasal cavity that help visualize the physiologic impact of alterations in intranasal structures. Therefore, it becomes possible to quantitatively measure, and visually appreciate, the airflow pattern (laminar or turbulent), velocity, pressure, wall shear stress, particle deposition, and temperature changes at different flow rates, in different parts of the nasal cavity. The effects of both existing anatomical factors, as well as post-operative changes, can be assessed. With recent improvements in CFD technology and computing power, there is a promising future for CFD to become a useful tool in planning, predicting, and evaluating outcomes of nasal surgery. This review discusses the possibilities and potential impacts, as well as technical limitations, of using CFD simulation to better understand nasal airflow physiology.

The Optimal Number and Location of Discrete Radiant Heaters in Enclosures with the Participating Media Using the Micro Genetic Algorithm

A methodology for solving inverse radiation heat transfer problems when design variables are discrete is presented. An optimization method is used to find the best number and location of discrete equal power heaters to produce the desired temperature and heat flux distribution over the design surface of radiant enclosures. The medium is assumed to be absorbing, emitting, and scattering. Isothermal and radiative equilibrium media are considered. The modified discrete ordinates method, that mitigates ray effects caused by discontinuities or abrupt changes of wall temperature, is employed to solve the radiative transfer equation. The micro genetic algorithm is used to minimize objective function.

Effect of Heat-Reflective Insulation Coating on Exterior Walls of Building Energy Consumption in Summer

To discuss the effect of energy saving of heat-reflective insulation coating on exterior walls of building, the wall temperature change of two test rooms with heat insulation or not in Hangzhou has been monitored in summer, and the heat flux data of east, south and west walls were analyzed. The results indicate that the temperature of outside wall surface with the heat reflective insulation coating is reduced effectively, and the maximum temperature change was about 9, 10°C. There is a positive proportional between the mean daily air temperature and heat gain that the heat gain increases with the mean daily air temperature. By calculating, the higher the air temperature, the more the cooling effect of coating in summer. But the higher the mean daily temperature do not mean that the more heat gain in summer. The result shows the energy saving effect of the heat insulation coating on building wall was very obvious in summer in Hangzhou.

Stability of a thermal boundary layer in the presence of vibration in weightlessness environment

Several weightless experiment with supercritical fluids have shown that thermal boundary layers can be destabilized when submitted to a harmonic vibration. A study of the phenomenon is given here in a regular fluid during a sudden change of wall temperature in the presence of harmonic tangential vibrations and under weightlessness. A semi-infinite space is filled with a fluid and bounded by a flat wall oscillating in its plane. For this configuration, a state with the fluid velocity parallel to the wall is possible but this fluid motion does not influence the heat transfer. Then the propagation of thermal waves can be described by classical relations. The stability of this state is studied under the assumption of a “frozen” temperature profile. The vibration frequency is assumed to be high such that the viscous boundary layer thickness is small in comparison with the thermal boundary layer thickness. The calculations show that the instability develops when the thickness of the thermal boundary layer attains a critical value. The wavelength of the most dangerous perturbations is found to be about twice the critical thermal boundary layer thickness.

MODELLING ON DIRECTIONAL SOLIDIFICATION OF SUPERALLOY BLADES WITH FURNACE WALL TEMPERATURE EVOLUTION

Ni-based superalloy turbine blades produced by Bridgman directional solidification technology are widely used in both aeronautic and energy industries as key parts of the gas turbine engines. Because of existence of complex heat radiation between the shell surface and the furnace wall, precise control of the temperature distribution within the blade is a challenging task. A modified model based on the Monte Carlo ray tracing method was proposed for the three dimensional temperature simulation of the turbine blades during directional solidification process, in which the furnace wall temperature evolution was considered and calculated. Ray refinement in normal direction was applied to improve the heat radiation calculation precision. Three dimensional finite differential grids for turbine blades and two dimensional differential grids for furnace wall were used together to increase simulation efficiency and save memory consumption. Heat transfer calculation of the blades with the modified model was performed and compared with that of the simplified model in which the furnace wall temperature was treated as constant. Experiments were carried out to validate the proposed model in this paper. It was demonstrated that the modified model revealed the furnace wall temperature change during the withdrawal process and its impact on the blade, and simulated the temperature distribution of the turbine blade with a higher accuracy.

Unsteady Natural Convection Flow in a Square Cavity Filled with a Porous Medium Due to Impulsive Change in Wall Temperature

Unsteady natural convection flow in a two- dimensional square cavity filled with a porous material has been studied. The flow is initially steady where the left- hand vertical wall has temperature T-h and the right- hand vertical wall is maintained at temperature T-c ( T-h > T-c) and the horizontal walls are insulated. At time t > 0, the left- hand vertical wall temperature is suddenly raised to (T-h) over bar ((T-h) over bar > T-h) which introduces unsteadiness in the flow field. The partial differential equations governing the unsteady natural convection flow have been solved numerically using a finite control volume method. The computation has been carried out until the final steady state is reached. It is found that the average Nusselt number attains a minimum during the transient period and that the time required to reach the final steady state is longer for low Rayleigh number and shorter for high Rayleigh number.

Axial heat conduction effects in the entrance region of circular ducts

This paper describes the transport of thermal energy within a small distance after an abrupt wall temperature change in a circular duct. In general, the axial conduction becomes significant when the Peclet number is small. The results indicate that the inclusion of axial conduction in the fluid substantially increases the wall heat flux at near the thermal inlet location. The exact series solution leads to a modified Graetz type problem. This exact solution is augmented by an asymptotic solution describing the wall heat flux near the thermal entrance location.

Axial heat conduction effects in the entrance region of parallel plate ducts

Exact series solutions for the computation of temperature in parallel plate channels and circular passages are well known. The inclusion of the contribution of axial conduction leads to a set of modified Graetz type problems for these fluid passages. The emphasis of this paper is the study of the asymptotic variations of wall heat flux values adjacent to the thermal entrance location for parallel plate ducts. The acquired results show interesting variations for different values of the Peclet number near the thermal entrance location. This study reports the unique variations of the wall heat flux values near the location where the wall temperature changes as the Peclet number changes.

Semi-analytical solution of the extended Graetz problem for combined electroosmotically and pressure-driven microchannel flows with step-change in wall temperature

Analytical solutions are obtained for the temperature and Nusselt number distribution in the thermal entrance region of a parallel plate microchannel under the combined action of pressure-driven and electroosmotic transport mechanisms, by taking into account the effects of viscous dissipation, Joule heating and axial conduction simultaneously, in the framework of an extended Graetz problem. Step changes in wall temperature are considered to represent physically conceivable thermal entrance conditions. The solution of the temperature distributions at the various channel sections essentially involves the determination of a set of eigenvalues and eigenfunctions corresponding to a Sturm Liouiville problem with non self-adjoint operators. The resultant eigenfunctions are non orthogonal in nature, and are obtained in the forms of hypergeometric functions. Parametric variations on the effects of the relative strengths of the pressure gradients and the electric field, ratio of the rate of heat generation to the rate of wall heat transfer, and the Peclet number are analyzed in details, in terms of their influences on the temperature field as well as the Nusselt number distribution.

Transient natural convection and heat transfer during the storage of granular media

Transient heat transfer in an originally isothermal cylinder filled with a porous medium after sudden change of wall temperature is studied experimentally and computationally. Lab-scale experiments with water as the interstitial fluid are used in order to imitate the conditions prevailing in large, air-filled industrial silos. The proposed model assumes isotropy of the porous medium, local thermal equilibrium between the phases, Darcy flow and applicability of the Boussinesq approximation. Its predictions are in satisfactory agreement with the experimental results. Simulations reveal the role of dimensionless parameters like the modified porous media Rayleigh number and the cylinder aspect ratio. A criterion for neglecting the influence of natural convection on heat transfer is established.

Buoyant Flow with Viscous Heating in a Vertical Circular Duct Filled with a Porous Medium

Combined, forced, and free flow in a vertical circular duct filled with a porous medium is investigated according to the Darcy–Boussinesq model. The effect of viscous dissipation is taken into account. It is shown that a thermal boundary condition compatible with fully developed and axisymmetric flow is either a linearly varying wall temperature in the axial direction or, only in the case of uniform velocity profile, an axial linear-exponential wall temperature change. The case of a linearly varying wall temperature corresponds to a uniform wall heat flux and includes the uniform wall temperature as a special case. A general analytical solution procedure is performed, by expressing the seepage velocity profile as a power series with respect to the radial coordinate. It is shown that, for a fixed thermal boundary condition, i.e., for a prescribed slope of the wall temperature, and for a given flow rate, there exist two solutions of the governing balance equations provided that the flow rate is lower than a maximum value. When the maximum value is reached, the dual solutions coincide. When the flow rate is higher than its maximum, no axisymmetric solutions exist.

A study of nucleate boiling and critical heat flux with EHD enhancement

The paper describes results from an experimental and theoretical study of the effect of an electric field on nucleate boiling and the critical heat flux (CHF) in pool boiling of R123 at atmospheric pressure on a horizontal wall with a smooth surface. Two designs of electrode (parallel rods and wire mesh) were used. The experimental data exhibit some differences from the data obtained by other researchers in similar experiments on a wall with a different surface finish and with a slightly different design of wire mesh electrode. The hydrodynamic model for EHD enhancement of CHF cannot reconcile the differences. A theoretical model has been developed for the growth of a single vapour bubble on a superheated wall in an electric field, leading to a numerical simulation based on the level-set method. The model includes matching of sub-models for the micro- and macro-regions, conduction in the wall, distortion of the electric field by the bubble, the temperature dependence of electrical properties and free-charge generation. In the present form of the model, some of these effects are realised in an approximate form. The capability to investigate dry-spot formation and wall temperature changes that might lead to CHF has been demonstrated.

A numerical model for the thermocapillary flow and heat transfer in a thin liquid film on a microstructured wall

PurposeThis paper aims to study thermocapillarity‐induced flow of thin liquid films covering heated horizontal walls with 2D topography.Design/methodology/approachA numerical model based on the 2D solution of heat and fluid flow within the liquid film, the gas above the film and the structured wall is developed. The full Navier‐Stokes equations are solved and coupled with the energy equation by a finite difference algorithm. The movable gas‐liquid interface is tracked by means of the volume‐of‐fluid method. The model is validated by comparison with theoretical and experimental data showing a good agreement.FindingsIt is demonstrated that convective motion within a film on a structured wall exists at any nonzero Marangoni number. The motion is caused by surface tension gradients induced by temperature differences at the gas‐liquid interface due to the spatial structure of the heated wall. These simulations predict that the maximal flow velocity is practically independent from the film thickness, and increases with increasing temperature difference between the wall and the surrounding gas. It is found that an abrupt change in wall temperature causes rupture of the liquid film. The thermocapillary convection notably enhances heat transfer in liquid films on heated structured walls.Research limitations/implicationsOur solutions are restricted to the case of periodic wall structure, and the flow is enforced to be periodic with a period equal to that of the wall.Practical implicationsThe reported results are useful for design of the heat transfer equipment.Originality/valueNew effects in thermocapillary convection are presented and studied using a developed numerical model.

Effects of viscous dissipation and fluid axial heat conduction on heat transfer for non-Newtonian fluids in ducts with uniform wall temperature: Part II. Annular ducts

The present paper, which is an extension of a previous study [O. Jambal, T. Shigechi, D. Ganbat, S. Momoki, Int. Commun. Heat Mass Transf. 32(9) (2005) 1165] on laminar heat transfer to non-Newtonian fluids in parallel plates and circular ducts, deals with concentric annular ducts subjected to a step change in wall temperature. A finite-difference scheme is applied to determine the velocity and temperature fields. Developing Nusselt numbers are graphically shown for various Brinkman numbers and Peclet numbers and the effects of viscous dissipation, fluid axial heat conduction, non-Newtonian behavior together with the radius ratio effect on heat transfer are discussed.