Transient Heat Transfer Effects Research Articles

Experimental study of thermally enhanced recovery of high-viscosity DNAPL in saturated porous media under non-isothermal conditions

Thermal enhancement is known to be an efficient way to decrease the residual saturation of some common dense non-aqueous phase liquids (DNAPLs) after pumping. However, the effect of transient heat transfer during the recovery of a high-viscosity contaminant, such as coal tar, in highly permeable porous media is still unknown. A 2D tank experimental setup allowing monitoring of temperature and saturation fields during DNAPL pumping has been developed. Experiments were run under isothermal and non-isothermal conditions, at low and high flow rates. We investigated the presence of viscous fingering and how that influences the shape of the cone of depression, as well as the residual saturation. The saturation fields show that less viscous fingering occurs in pre-heated cases and that heating increases the recovery efficiency. Increasing the temperature increases the critical velocity and the viscosity ratio and helps to stabilize the interface between the non-wetting and wetting phase. Observations were first made on an oil and ethanol fluid pair because its properties were known, before extending the experiments to a coal tar and water fluid pair. Residual oil saturation after pumping was decreased by 6–16% in all pre-heated conditions. Pumping at low flow rate in these conditions leaves the smallest oil residual saturation (20%) after pumping. A low flow rate increases the recovery efficiency by reducing viscous fingering and by spreading the generated heat to a larger part of the tank. Finally, results on coal tar pumping show that the high thermal conductivity of water helps in keeping the temperature high during pumping. The residual coal tar saturation was reduced from 40% at 20 °C to 28% when pre-heating the tank. Operating at a low flow rate and with a uniform temperature is the key to recovering the highest amount of a viscous DNAPL such as coal tar from the soil and satisfying cleanup goals when using thermally enhanced pumping.

An analytical full-scale model to predict thermal response in boreholes with groundwater advection

Borehole heat exchanger (BHE) is an important component in the ground coupled heat pump (GCHP) system. Based on the theories of composite medium line-source and the moving line-source, an analytical full time-scale model has been derived to predict the performance of BHE by considering the coupled effects of transient heat transfer in the borehole, the horizontal groundwater around the borehole and the axial heat conduction along the borehole. The full-scale model is validated by the experimental data and the numerical results. The validated model is applied to predict fluid temperature in the cases (i) with unit load, (ii) with annual building loads and (iii) with multiple boreholes. Based on the simulation results, the impacts of time length, axial conduction, groundwater flow and locations of boreholes are discussed in detail.

Exact thermoelastic analysis of a thick cylindrical functionally graded material shell under unsteady heating using first order shear deformation theory

AbstractIn this article, a new analytical formulation is presented for an axisymmetric thick‐walled functionally graded material cylinder with power‐law variation in mechanical and thermal properties under transient heating using first order shear deformation theory. Equilibrium equations are derived by virtual work principles and the energy method. The unsteady heat conduction equation is solved using the method of separation of variables, generalized Bessel functions, and an Eigen‐function method. Validation of the analytical solutions is conducted with a finite element method. The effects of time on stress and displacement distribution are studied in detail. The numerical values used in this study are selected based on earlier studies. The influence of effect of transient heat transfer on heterogeneous thick‐walled cylinder elasticity is clearly demonstrated. In particular, the significant influence of time and the heterogenous constant on radial displacement, hoop stress, and temperature distributions is computed. The study is relevant to rocket chamber thermomechanics, propulsion duct thermophysical design, industrial thermal storage systems, and so forth.

Effects of Transient Heat Transfer on Compressor Stability

The effects of heat transfer between the compressor structure and the primary gas path flow on compressor stability are investigated during hot engine re-acceleration transients. A mean line analysis of an advanced, high-pressure ratio compressor is extended to include the effects of heat transfer on both stage matching and blade row flow angle deviation. A lumped capacitance model is used to compute the heat transfer of the compressor blades, hub, and casing to the primary gas path. The inputs to the compressor model with heat transfer are based on a combination of full engine data, compressor test rig measurements, and detailed heat transfer computations. Nonadiabatic transient calculations show a 8.0 point reduction in stall margin from the adiabatic case, with heat transfer predominantly altering the transient stall line. 3.4 points of the total stall margin reduction are attributed to the effect of heat transfer on blade row deviation, with the remainder attributed to stage rematching. Heat transfer increases loading in the front stages and destabilizes the front block. Sensitivity studies show a strong dependence of stall margin to heat transfer magnitude and flow angle deviation at low speed, due to the effects of compressibility. Computations for the same transient using current cycle models with bulk heat transfer effects only capture 1.2 points of the 8.0 point stall margin reduction. Based on this new capability, opportunities exist early in the design process to address potential stability issues due to transient heat transfer.

Jet Engine Gas Path Analysis Based on Takeoff Performance Snapshots

Current engine condition monitoring (ECM) systems for jet engines include the analysis of on-wing gas path data using steady-state performance models. Such data, which are also referred to as performance snapshots, usually are taken during cruise flight and during takeoff. Using steady-state analysis, it is assumed that these snapshots have been taken under stabilized operating conditions. However, this assumption is reasonable only for cruise snapshots. During takeoff, jet engines operate in highly transient conditions with significant heat transfer occurring between the fluid and the engine structure. Hence, steady-state analysis of takeoff snapshots is subject to high uncertainty. Because of this, takeoff snapshots are not used for performance analysis in current ECM systems. We quantify the analysis uncertainty by transient simulation of a generic takeoff maneuver using a performance model of a medium size two-shaft turbofan engine with high bypass ratio. Taking into account the influence of the preceding operating regimes on the transient heat transfer effects, this takeoff maneuver is extended backward in time to cover the aircraft turnaround as well as the end of the last flight mission. We present a hybrid approach for thermal calculation of both the fired engine and the shutdown engine. The simulation results show that takeoff derate, ambient temperature, taxi-out (XO) duration and the duration of the preceding aircraft turnaround have a major influence on the transient effects occurring during takeoff. The analysis uncertainty caused by the transient effects is significant. Based on the simulation results, we propose a method for correction of takeoff snapshots to steady-state operating conditions. Furthermore, we show that the simultaneous analysis of cruise and corrected takeoff snapshots leads to significant improvements in observability.

Effect of Transient Heat Transfer of a Condenser on a Cascade Heat Pump Performance

Transient heat transfer characteristics of a condenser of a cascade heat pump are investigated by comparing the experimental result and the prediction from a mathematical model. In the experiment part, the cascade heat pump consists of the low-temperature refrigeration cycle using R22 and the high-temperature heat pump cycle using R134a.The transient effect is presented by circulating water to receive the rejection heat from the condensing refrigerant of the cascade heat pump causing the rise of the hot water temperature. On the other hand, a mathematic model of the condensation heat consisting of the heat transfer models in the two-phase mixture region and the superheated vapor region is developed to compare the results. The effect of the hot water flow rate on the hot water temperature and the heat transfer rate is examined. The result indicates that the hot water temperature continuously increases with time. The condensation heat rapidly increases at the beginning and gradually increases with time. The result obtained from the model prediction of the condensation heat is similar to the experimental result except the initial and final period. The discrepancy between these two results is because of the transient behavior at beginning and the existent of the subcooled region.

Numerical investigation into dynamic responses of RC columns subjected for fire and blast

Structures may be exposed to fire and blast due to accidents (i.e. explosion of flammable gas in industrial structures) or terrorist attacks during the service life. Performances of RC structures subjected to extreme conditions of fire and blast, thus, have drawn much attention from academia. In this paper, the coupling effect of high temperature and high strain rate in concrete was firstly studied based on the experimental data to improve the damage plasticity concrete model in ABAQUS. Secondly, the transient heat transfer effects in different fire scenarios and following fire resistances of RC columns with constant axial forces were numerically investigated on the basis of the improved concrete model, which are validated by the corresponding test data, and the residual axial loading capacity of RC columns was quantitatively calculated. By incorporating the different merits of implicit algorithm applied to heat transfer analyses and explicit algorithm usually used in blast analyses, a numerical approach to analyze the responses of RC columns subjected to the coupling loadings of fire and blast was finally developed. Mid-span displacements and damage of the RC columns subjected to fire and explosions were quantitatively calculated and discussed. The proposed approach was demonstrated to be effective in predicting the responses of RC structures subjected to coupling loadings of fire and blast.

Transient Wax Gel Formation Model for Shut-In Subsea Pipelines

Physics of wax gel formation during shut-in is analyzed and described over a cross-section of a typical subsea pipeline. Two regions are identified during this process: the liquid and gel regions. Phase transition is assumed to occur at the liquid-gel interface. Unsteady-state heat and mass transfer models are proposed for each region. Two diffusion streams are evaluated: the dissolved wax molecules moving from the pipe center toward the wall due to temperature gradient and subsequently concentration gradient and the wax molecules diffusing from the liquid-gel interface into the gel deposit. This model is essentially the modification of the model given by Bhat et al. [1] which considered transient heat transfer and neglected mass transfer of wax molecules through the gel deposit and the model by Singh et al. [2] which considered transient mass transfer of molecules with carbon numbers higher than the` critical carbon number (CCN) necessary for wax diffusion into gel deposit but did not consider transient heat transfer effects during the cooling process. This paper presents a transient-state formulation circumventing the limitations of these previous models and better represents the true cooling and gelation process occurring in a shut-in subsea pipeline filled with waxy crude.

Modeling blowdown of pipelines under fire attack

AbstractThe development of a mathematical model for simulating the blowdown of pressurized hydrocarbon conveying pipelines under fire attack is described. The model is based on the resolution of the conservation equations using the method of characteristics. It accounts for real fluid behavior, pipeline mechanical strength, as well as phase and flow dependent transient heat transfer effects, and frictional pressure losses. Failure is assumed to occur when any one of the simulated triaxial thermal and pressure stresses in the pipeline wall exceed its ultimate tensile strength. The application of the model to a hypothetical example involving localized fire attack on an isolated natural gas pipeline reveals that the pipeline fails because of large thermal stresses on the outer surface in addition to the internal pressure stresses. The efficacy of emergency depressurization using different diameter relief valves as a means of protecting the pipeline mechanical integrity during fire attack is also quantitatively investigated. © 2007 American Institute of Chemical Engineers AIChE J, 2007

Effect of transient heat transfer on ignition of solid particles

Ignition and combustion of solid particles are the issues of interest for many industrial applications. When simulating ignition and combustion of solid particles using available standard (ST) models, a number of simplifying assumptions are usually adopted, which are not always justified. For example, for calculating heat flux to particle surface, the Newton law is often applied with the heat flux proportional to the difference between the gas temperature and the mean particle temperature. However, Newton law is known to be valid only for steady-state heat transfer. Moreover, the actual heat flux is determined by the particle surface temperature rather than its mean temperature. The objective of this work is to develop a new particle-heating model with the correction factors to the Newton law taking into account transient heat transfer to a particle and nonuniform temperature distribution inside the particle. It was shown that the new particle-heating model correlates much better with detailed numerical calculations than the ST model. The transient heating effects were shown to be important for the problem of solid particle ignition in the oxidizer gas.

Three-Dimensional FE–NCV Modeling of Thermoplastic Composites Pultrusion

A three-dimensional (3D) model for the simulation of the thermoplastic pultrusion process is developed. The simulation procedure and algorithms are based on the Finite Element–Nodal Control Volume (FE–NCV) approach. For illustration purposes, APC-2 CF–PEEK is used as a candidate material along with multiheater steel die assemblies. Heat transfer in the die and the composite is modeled using FEs whereas NCVs are used to model crystallization kinetics and heat of reaction for PEEK. The developed simulation procedure is validated using available data. Subsequently, versatility of the model for simulating various types of die assembly and transient heat transfer effects on crystallization is demonstrated.

A study on thermomechanical deformation of elastic beam with embedded shape memory alloy wires

The bending deformation of an elastic beam with eccentrically embedded shape memory alloy (SMA) wires under the activation of electrical current has been investigated. A procedure, which accounts for the effect of transient heat transfer and gives rise to the beam bending responding to prescribed input electrical current profile is used for analysis. It is found that influences of ambient temperature, electric current intensity and prestrain of SMA on the bending deformation of beam are significant.

Transient heat transfer effects on the pseudoelastic behavior of shape-memory wires

Experimental results of a displacement-controlled elongation of a shape-memory wire of nickel-titanium are presented. It is observed that the hysteretic strain-stress curves depend strongly on the strain rates at which the wire is extended. A theoretical model is proposed to explain this phenomenon. This model couples the fully time-dependent heat transfer in the wire to its quasi-static mechanical behavior through the temperature dependence of the transformation stress of the alloy. It accounts quantitatively for experimentally observed changes in the pseudoelastic hysteresis. The model presented here is different from others proposed in the literature, as it does not make use of a kinetic relation and accounts for the observed changes in the pseudoelastic hysteresis without parameter fitting. The results show that a model consisting of a single moving austenite-martensite interface is sufficient to predict the response of the wire over several decades of strain rate.

Thermal propagation of normal zone with increasing temperature level in helium-cooled and high temperature superconductors

Normal zone propagation is investigated above the maximum equilibrium current l ∗, where a superconductor has no stable state in the normal region. This monostable thermal wave is shown to have constant front velocity although its crest grows infinitely with time. Various monostable regimes are analysed by numerical simulation of zone propagation both in helium-cooled and in high T c superconductors (HTSC), and the velocity-current dependences are obtained. Unlike in HTSC, transition to monostable propagation in helium-cooled superconductors is accompanied by jump in velocity at l = l ∗. A constant coefficient model for HTSC is presented which gives analytical expressions for monostable zone velocity. Over a wide range of currents the velocity is given by universal formula dependent on normal substrate properties only. The effect of transient heat transfer is analysed.

Effect of heat transfer on the performance of geothermal wells

The effect of heat losses on the performance of geothermal wells is investigated by means of a one-dimensional mathematical model, which includes some novel features for the calculation of the pressure drop along the well. One of two existing correlations is selected for the friction gradient in the slug-flow regime according to the value of a new dimensionless parameter. In addition, a modified version of Tachimori's correlation is used for the mist-flow regime. The model also accounts for the effect of transient heat transfer between the well and the surroundings by accommodating temporal variations of the temperature distribution within the surrounding rock. The capability of the model is assessed by comparing the calculated results with existing sets of experimental data. The model predictions are in good agreement with the data. It is found that heat transfer effects are particularly significant when the well is running outside the range of nominal operating conditions, e.g. when the well is throttled. Comparison of the model predictions with those obtained applying Ramey's method, shows the advantages of using the present approach under particular circumstances.

Transient Response and Disturbance Growth in Vertical Buoyancy-Driven Flows

The basic physical characteristics of flow response to a changed bounding-surface energy input is reviewed. Response regimes are delineated in terms of locally laminar, unstable, transition, and turbulent flows and dimensionality and regimes intermediate to the others. Very large transient heat transfer effects arise. A key general question is how such regimes become unstable and progress to turbulence. Transient disturbance growth analysis is very difficult and few results are available. This paper develops a general and purely numerical formulation for two-dimensional transient response and disturbance growth. It relies on imposed random flow and associated temperature and disturbance motion pressure disturbances. It applies to any boundary region regime and is specialized here to disturbance growth in a developing buoyancy-driven flow, as an example.

Propagating Normal Zones and Transient Heat Transfer Effects in Superconducting Wires

Theoretical and experimental data of normal zone propagating velocities along superconducting wires immersed in a normal helium bath are presented. Longitudinal destruction of the superconductivity arises from thermal instabilities along the superconducting wire. Propagating normal zone velocities are thus strongly dependent on the thermal transfer mechanism between the helium bath and the superconducting wire. Transient heat transfer effects are necessary to obtain good agreement between experiment and theory. The used transient heat transfer coefficient depends on the bath temperature and on the sample resistivity as it can be seen from our results.

Effects of transient heat transfer to liquid helium on steady propagation velocity of normal zones in superconducting wires

The transient cooling mechanism of liquid helium which affects the propagation velocity of normal zones in bere superconducting composite wires is discussed in relation to the transit time of a zone front region. Previously the propagation velocity was calculated using the transient cooling mechanism either due to metastable nucleation on the heated wire or due to thermal diffusion in vapour films covering the wire. In the present paper, both of the above-mentioned cooling mechanisms are considered for evaluating the propagation velocity in the manner that one mechanism switches to the other at a certain time. The propagation velocity is obtained by solving numerically a one-dimenional heat balance equation on the basis of the present mechanisms. Comparison of the present results with our experimental data shows good agreement.

Transient heat transfer model for normal zone propagation. Part 1 — theory of a bare helium-cooled superconductor

The paper presents a physical model, involving no adjustable parameters of transient heat transfer for normal zone propagation in a composite helium-cooled superconductor. Evaluations are given showing that the main contribution to transient heat flux is made by transverse heat conduction in the coolant. When the zone front passes through a fixed point, the temperature at that point increases drastically, a great temperature gradient appears in coolant which results in transient cooling of the superconductor. The zone propagation equation becomes an integro-differential with a convolution integral. The transient heat transfer intensity is characterized by the introduced dimensionless parameter ϵ. The problem has been solved analytically for the model with constant coefficients and jump-wise heat release. Formulae have been obtained for the propagation and recovery velocities describing the effect of transient heat transfer (via parameter ϵ).

Propagation of normal zones in composite superconductors

This paper describes calculations of propagation velocities of normal zones in composite superconductors. Full account is taken of (1) current sharing, (2) the variation with temperature of the thermal conductivity of the copper matrix, and the specific heats of the matrix and the superconductor, and (3) the variation with temperature of the steady-state heat transfer at a copper-helium interface in the nucleate-boiling, transition, and film-boiling ranges. The theory, which contains no adjustable parameters, is compared with experiments on bare (uninsulated) conductors. Agreement is not good. It is concluded that the effects of transient heat transfer may need to be included in the theory to improve agreement with experiment.