Transient Heat Transfer Coefficients Research Articles

Inverse Estimation of Swirl Cooling Heat Transfer Coefficient

Swirl cooling is one of the most widely studied techniques in the field of heat transfer enhancement. This paper presents a novel, non-intrusive, and rather inexpensive inverse method to estimate the local heat transfer coefficient along the swirl chamber. First, the direct problem of two-dimensional heat conduction inside the chamber’s wall is solved numerically. The optimal number of sensors, their axial and radial positions, and other parameters concerning the inverse algorithm are determined using simulated experiments. The experimental setup is built based on these parameters. Transient temperature readings from several thermocouples embedded inside the chamber wall are fed to an inverse algorithm based on the conjugate gradient method. This algorithm takes advantage of an adjoint differential equation and the sensitivity differential equation to calculate the gradients and optimal step sizes, respectively. The local transient heat transfer coefficient is estimated for three Reynolds numbers along four straight lines parallel to the axis of the chamber.

A transient formulation of entropy and heat transfer coefficients of Newton's cooling law with the unifying entropy potential difference in compressible flows

The original Newton's law of cooling is considered an ideal convection model of a point source ignoring conduction, but its unknown convection conductance and inconsistent thermal potential difference make it difficult to further elucidate the transient heat transfer mechanism of convection because of the no-slip condition at the interface between a body and a fluid. For this purpose, both the convective entropy transfer theory and the hypothesis of the isothermal velocity sublayer are developed to recast Newton's law of cooling into a standard Ohm's law form via the unified entropy driving force and the unambiguous entropy transfer coefficient. An explicit expression for the transient heat transfer coefficient is presented theoretically and experimentally in compressible flows by virtue of the product of the free-stream velocity, the volumetric thermal capacity of the fluid, and the near-wall velocity constant. The unified thermal potential difference is established between the body temperature and the total temperature (reduced to the ambient temperature for incompressible fluids). The physical mechanism of convection mode in the original Newton's cooling law is reasonably explained by the entropy transfer theory and entropy transfer efficiency, and the formulae for the entropy flux vector driven by the standard entropy potential difference and the rate of entropy generation are obtained. The analytical heat transfer coefficients as well as the standard thermal driving forces are validated by comparison with the forced convection experiment with a maximum error of 4.6 % in incompressible flows and the previous Newton's benchmark cooling experiments with the maximum error of 6.7 % in compressible flows. The effects of radiation on the total heat transfer coefficient are also analyzed quantitatively.

Enhanced heat transfer in microgravity from asymmetric sawtooth microstructure with engineered cavities

Nucleate pool boiling in microgravity is influenced by stagnant vapor bubble dynamics, leading to early dryout of the heated surface compared to terrestrial conditions. The Asymmetric Sawtooth and Cavity-Enhanced Nucleation-driven Transport (ASCENT) investigation studied an engineered microstructure in a nucleate pool boiling setup aboard the International Space Station (ISS) from November 2022-January 2023. The experiment was housed in the Microgravity Science Glovebox (MSG) and used degassed FC-72 as the test liquid. The engineered microstructure consisted of repeating mm-sized 60°-30° sawtooth structures located within a hermetically sealed square ampoule as the test chamber. Experimental investigations were conducted to explore a range of input heat fluxes, ranging from 0.5 to 2.3 W/cm². The resulting vapor dynamics and heat transfer were compared against a flat baseline surface. Both surfaces contained 250 µm square cavities spaced 1-mm apart. The constrained dimensions of the square chamber severely influenced vapor bubble dynamics across the heated surface. The presence of a liquid layer beneath vapor slugs on the microstructure was observed, in contrast to the baseline surface, where no liquid layer was visually detected. The transient heat transfer coefficient from the microstructure was higher than the baseline surface as the vapor bubbles grew larger at constant heat flux due to liquid film access provided by the asymmetric ridges on the microstructure. A significant increase in the heat transfer coefficient was observed at 1.8 W/cm2, potentially due to vigorous nucleation in the observed liquid pockets along the microstructure, increasing the heat transfer coefficient from 2900 to 6200 W/m2K. Nucleation was observed at heat fluxes as low as 0.5 W/cm2 from the engineered cavities, and a quasi-steady heat transfer coefficient of 2200 W/m2K was obtained at 2.3 W/cm2 from the microstructure surface. The quasi-steady analysis also indicated that both surfaces performed similarly in microgravity and terrestrial gravity in the same experimental setup. The results demonstrate that the liquid film dynamics underneath vapor slugs influence heat transfer to a large extent in microgravity conditions.

Characteristics of the transient heat transfer of impinging flames and correlation analysis using a new characteristic velocity under CI engine-like conditions

Understanding the transient heat transfer mechanism of impinging flames is a crucial pathway for further improving the thermal efficiency of already efficient compression ignition (CI) engines. In this paper, the investigation of the transient heat transfer of wall-impinging flames was performed in a high-pressure constant-volume vessel. Fast-response thermocouples were installed in the impinging wall to record the transient heat flux. Two-color pyrometry was employed to estimate the mean temperature inside the flame region. Firstly, the transient heat transfer characteristics were investigated under varied ambient density, oxygen concentration, temperature, and injection pressure conditions. The optical flame velocity calculation method was applied to this extensive dataset to develop a heat transfer correlation between Nu and Re with which transient heat transfer coefficients were calculated and compared with the experimental results. From this analysis, cumulative fuel injection velocity was developed as the new characteristic parameter to characterize the transient heat transfer of the wall-impinging flames. Results indicate that the effect of fuel injection pressure on the heat flux is more significant than that of ambient gas conditions, with higher injection pressure causing higher heat flux through the impinging wall. No obvious linear tendency of transient Nu and Re was found when using the optical measurement results of the wall-impinging flame as the characteristic parameter. Instead, the cumulative fuel injection velocity shows a strong linear trend of transient Nu and Re during the transient heat transfer processes of the wall-impinging flame. Moreover, the heat transfer coefficients from the new cumulative fuel injection velocity well fit the experimental results.

Reconstruction of transient convective heat transfer coefficients in heat transfer of gun barrels with variable coefficients

On the basis of the conjugate gradient method, an inversion model of the transient convective heat transfer coefficient between the bore surface and the gunpowder gas is presented. In this study, the measured temperature at a certain depth from the bore surface is employed as input. The variation of the thermophysical properties of gun steel with respect to temperature is taken into consideration. After verifying the feasibility of the model with constant convective heat transfer coefficients, it is applied to the reconstruction of transient convective heat transfer coefficients that consistent with the classical interior ballistic laws. Results indicate that decreasing the measurement error and increasing the number of measurement points can effectively improve the performance of the model. As well, measurement points that placed further away from the inlet boundary offer superior accuracy. When studying the inverse heat transfer problem of the gun barrel, it is found that setting the monitoring time to be twice the duration of the interior ballistic period may guarantee sufficient input data. The inversion model may offer a promising theoretical framework for indirect measurement manner of the bore surface temperature of gun barrel.

Stagewise melting heat transfer characteristics of phase change material in a vertically placed rectangular enclosure

In this work, the stagewise melting heat transfer properties of lauric acid in a side-heated rectangular enclosure are investigated by using a numerical model validated by a published experimental study. During the discussion, the conduction process is treated as stage I, the processes of mixed convection–conduction and convection are collectively treated as stage II, and the shrinking solid process is treated as stage III. The effects of the enclosure geometry and the thermal conditions encountered by the lauric acid on heat transfer are discussed, particularly the transition points between different adjacent stages and the heat transfer characteristics during each stage. Increasing the height delays the transition point between stages I and II, but advances it between stages II and III. Increasing the width has no effect on the transition point between stages I and II, whereas it delays the transition point between stages II and III. Increasing the wall temperature and the initial temperature leads to an advancement of the two transition points. Dimensionless correlations are summarized for the transition points between adjacent stages, the melting completion times for the whole process, and the transient heat transfer coefficients and liquid fractions for each phase.

Experimental and Simulation Study on the Direct Contact Condensation of Saturated Steam on Moving Droplets at Sub-Atmospheric Pressure

The recovery of low-temperature steam is of great significance to the effective utilization of energy. Direct contact condensation (DCC) technology is an effective heat recovery method with a low initial investment. An evaluation of the direct contact condensation heat transfer technology of water droplets and low-temperature saturated steam (at an absolute pressure of 7.3–19.9 kPa) was performed using both experimental and computational approaches. In the experiment, an experimental device based on the weighing method was set up, and the direct contact heat transfer process between droplets with a normalized diameter of less than 21.7 and saturated pure steam at 40–60 °C was experimentally investigated. The transient condensation efficiency and heat transfer coefficient were calculated by the real-time mass variation. The rapid condensation stage was classified to discuss the effects of initial droplet diameter, pressure, temperature, and mass flow rate on the heat transfer process. A two-dimensional model was developed using computational fluid dynamics techniques and verified by experimental results. The results indicated that when the normalized diameter of the droplet is less than two, 1.6 is the optimal value for DCC. For droplets with a normalized diameter greater than two, the optimal droplet temperature and mass flow rate are 15 °C and 10 g/s, respectively.

Experimental investigation on flow boiling characteristics in offset strip fin channels under different sloshing conditions

In order to reveal the effects of sloshing movement on the performance of the plate-fin heat exchangers applied in floating liquefied natural gas platforms and environment control system of aircrafts, the flow boiling characteristics in offset strip fin channels of plate-fin heat exchangers under different sloshing conditions were investigated experimentally, covering rolling, pitching and swaying movements. The results show that, the rotational type movements enhance the transient heat transfer coefficient by a maximum of 21.1% at low vapor quality condition and weaken the transient heat transfer coefficient by a maximum of 27.7% at high vapor quality condition, while the transitional type movements enhance the heat transfer at high vapor quality. Fast Fourier transform method was used in the spectrum analysis of flow instabilities, and there are two peak frequencies caused by the inlet fluctuations, the distribution of the liquid phase and the enhanced turbulence. The time-average factors were proposed to evaluate the average heat transfer performance during the sloshing period. The ranges of sloshing time-average factors for rolling, pitching, and swaying conditions are 0.861 ∼ 1.079, 0.923 ∼ 1.061, 0.834 ∼ 1.064 and 0.994 ∼ 1.090, respectively. As the mass flux and sloshing period increase, the time-average factors trend to 1.0, and increase at the transitional sloshing movement. With the increasing amplitude, the time-average factors increase from 1 to a maximal value of 1.093. The correlations to predict the time-average two-phase heat transfer coefficients under rolling, pitching, swaying, and heaving conditions for offset strip fins have been developed, and the deviations are within ± 10 %.

Experimental study on transient heat transfer characteristics during the dropping of containment pressure

The reactor containment integrity and the pressure inside the containment are the most concern in the practical running process. To evaluate the transient heat transfer, non-equilibrium fraction (NEF) is proposed to represent the completed degree of transient condensation. An empirical correlation for NEF is developed and the fit error is within ±25% for 97% of results. Based on this, a simplified calculation model for transient heat transfer rate is also developed, then the transient condensation heat transfer coefficient (HTC) is obtained to characterize the intensity of transient heat transfer. The effect of mixed gas pressure, air mass fraction and temperature of cooling water on NEF and transient condensation HTC are discussed. The result indicates that the initial air mass fraction and initial gas-mixture pressure play an important role in transient-state heat transfer, and the influence of initial cooling water temperature can be ignored. This study aims to improve the understanding of transient-state heat transfer and provide guidance to practical engineering.

Experimental analysis of direct contact condensation during vertical injection of steam onto a subcooled water pool

Experiments have examined the phenomenon of direct contact condensation when steam is injected vertically into the subcooled water pool. The investigation is carried out by varying the steam mass flow rate and submergence depth of the steam injection pipe in the range of 10–50 kg/h and 1–13 cm, respectively. The behavior of the bubble that appeared at the pipe outlet, transient heat transfer coefficient, pressure variation in the steam injection pipe, and its associated frequency have been analyzed. The images captured by high-speed camera showed different bubble shapes. The overall cycle time of bubble evolution has decreased with an increase in the mass flow rate and increased with an increase in the pipe submergence depth. The time-averaged heat transfer coefficient increased with an increase in the mass flow rate and decreased with the rise of the pipe submergence depth. The pressure drop within the steam injection pipe shows the parabolic variation with an increase in the mass flow rate and is slightly influenced by the submergence depth due to changes in interfacial structures within the pipe. The peak frequency associated with the pressure has increased with an increase in the mass flow rate and decreased with an increase in the pipe submergence depth at higher mass flow rates. The fast Fourier transform of interfacial area of the larger bubble at the pipe outlet shows that the first peak frequency lies between 0.5 and 5 Hz, and the second peak frequency lies in the range of 25–30 Hz.

Transient heat transfer characteristics of submerged heat source under seismic excitation

In this experimental based study, experiments are carried out to investigate the transient heat transfer behavior from the horizontal heat source submerged in the partial filled water pool including the effect of submergence depth of heat source. Further, experiments are conducted and analyses have been made to investigate the heat transfer behavior under the seismic environment along with the consideration of seismic excitation parameters: natural frequency of liquid and amplitude of displacement.The transient heat transfer coefficient for natural convection and boiling region are evaluated with and without excitation cases. Experimental heat transfer coefficients were compared with the available correlations and found inadequate. Therefore, new correlations have been proposed for transient heat transfer coefficient for each region. For the seismic condition, separate correlation has been proposed to predict the vibrational heat transfer coefficient in case when strong sloshing occurs with wave breaking at the free surface. Proposed correlations are predicted quite satisfactory results.

Numerical study on the condensation characteristics of various refrigerants outside a horizontal plain tube at low temperatures

The condensation of refrigerants outside the tubes of liquefied natural gas (LNG) at low temperatures is the main factor affecting the performance of the LNG intermediate fluid vaporizer (IFV). In this paper, a CFD model is established to investigate the condensation heat transfer characteristics of refrigerants outside a horizontal plain tube at low temperatures over a wide range of wall subcooling. The model, based on the combination of the VOF multiphase model and Lee phase change model, agrees well with the experimental data. The transient condensation heat transfer coefficient is obtained and the periodic dripping characteristics of the condensate are analyzed. Due to the decrease of the interfacial mass transfer with the pressure decreasing, there is a peak of the periodic-averaged heat transfer coefficient with the variation of the saturation temperature, which is different from that of the Nusselt's prediction. The saturation pressures corresponding to the peak values of the periodic-averaged heat transfer coefficients at the subcooling of 10 °C are all in the range of 0.35–0.65 MPa for the four different refrigerants. In addition, the surface tension plays a positive role in the heat transfer process as the condensate film is pulled towards the wall and thinned. In view of the heat transfer performance along the whole LNG tube, dimethylether (DME) and butane can be considered as the intermediate fluids in the future design of the IFV by taking account of the comparison with six different refrigerants.

Numerical investigation on transient third-grade magnetized nanofluid flow and radiative convection heat transfer from a stationary/moving cylinder: nanomaterial and nanoparticle shape effects

In this study, a mathematical model is developed for analyzing the time-dependent magneto-convective flow and heat transfer characteristics of an electrically conducting (functional) third-grade Reiner-Rivlin non-Newtonian nanofluid from a moving or stationary hot cylinder in the presence of magnetic field and thermal radiation. A well-tested convergent Crank–Nicolson type finite difference algorithm is employed to solve the transformed, nonlinear boundary value problem. The Tiwari-Das nanofluid volume fraction model is adopted for nanoscale effects and the Rosseland algebraic flux model for radiative heat flux effects. It has been shown that the shape of nanoparticles remarkably contributes to the enhancement of heat transfer. Several metallic nanoparticle types such as Al2O3, Cu, and TiO2 are examined. It is found from the investigation that the viscoelastic nanofluid with TiO2 nanoparticles results in more heat transfer than the other nanoparticles. Lower velocity and higher temperature values are computed at transient conditions with a higher third-grade fluid parameter for the flow of nanofluid (Al2O3-SA). The plots of transient friction and heat transfer coefficients are visualized at the surface of a hot cylinder. The tabulated heat transfer coefficient is comparatively more for the moving cylinder than the stationary cylinder. Detailed validation of results of the numerical scheme with previous studies is included. The simulations find applications in coating deposition (enrobing) of magnetic nanomaterial at high temperatures, functional nanomaterial synthesis, etc.

Experimental investigation of lithium-ion cells ageing under isothermal conditions for optimal lifetime performance

Lithium-Ion cell ageing is sensitive to cell temperature. Previous studies have investigated ageing under adiabatic or controlled environmental temperature (i.e., isoperibolic) conditions. Notably, these conditions do not impose a uniform cell surface temperature (i.e., isothermal condition) or a controlled cooling rate, as an active Thermal Management System (TMS) would. This leads to a clear interdependence between charge/discharge rates and cell temperature, due to the uncontrolled cell temperature history. Consequently, the separate influence of these variables on the cell performance cannot be investigated. In this study, the ageing of a 300 mAh Lithium Cobalt Oxide (LCO Li-Ion) pouch cell under isoperibolic and isothermal conditions in the range of 0 °C - 40 °C is investigated. Each cycle comprises a CC-CV (constant current-constant voltage) charge of 1C and a CC discharge of 2C. Similar average ageing rates for isoperibolic and for isothermal conditions but at different reference temperatures were found. For example, an isoperibolic temperature condition of 25 °C yielded a similar degrading rate as an isothermal condition at 30 °C. This is mainly due to the effect of the cell self-heating (Joule heating) which increases the median operating temperature above that of the surroundings. These findings emphasise that uncontrolled cell thermal conditions lead to overall performance strongly dissimilar and randomly dependent on the transient heat transfer coefficient of isoperibolic TMS. Finally, an optimal isothermal condition that maximises the cell electrochemical efficiency and minimises its ageing is identified in the range of 25 °C-35 °C.

Transient experimental investigation of airside heat transfer in a crossflow heat exchanger

• Experimental analysis is performed to study the dynamic response of a heat exchanger. • Residence time is found to have a significant effect on the response time. • Transient air heat transfer coefficient is obtained by energy balance on the wall. • An empirical correlation of airside Nusselt number is developed. • The response time of both fluids was not instantaneous to the step imposed. The increase in energy consumption and the demand for effective thermal management systems necessitate the search for enhanced heat exchangers with high thermal performance, lower weight, and compact size. Crossflow heat exchangers are a key component in the thermal management system of many industries such as HVAC, automotive, etc. Transient analysis helps predict the behavior of heat exchangers when they experience variations in their operational conditions in terms of flow rate or temperature. This work experimentally studies the dynamic response of a cross flow heat exchanger to overcome the data scarcity in transient airside and presents the results for various cases of step changes in air mass flow rate. The effect of the step change on heat transfer is investigated and presented by airside Nu, fluids dimensionless temperature, airside Re, heat transfer rates, and Colburn j factor. Results show that the two fluids do not respond instantaneously to the step imposed. The magnitude of the step change has a dissimilar effect on the response time of each fluid. The response time increases with the increase in the step change. A generalized empirical correlation is obtained to predict the airside Nu. This study advances the control of heat exchangers by exploring their transient response due to step changes in one of the fluids inlet condition.

A modified sequential gradient-based method for the inverse estimation of transient heat transfer coefficients in non-linear one-dimensional heat conduction problems

In this communication, we introduce an online sequential implementation of a gradient-based method for reconstructing transient heat transfer coefficients in the context of non-linear one-dimensional heat conduction problems. Such a method employs a quasi-Newton updating strategy for computing the descent direction, in contrast with the traditional approach based on the conjugate gradient method. We denote the resulting procedure as the sequential quasi-Newton method (SQNM). The performance of the proposed algorithm was tested in the reconstruction of triangle-, sine-, and square-wave functions that models different transient heat transfer coefficients and compared with the results obtained using a standard sequential function specification method. The SQNM was capable of properly reconstruct the aforementioned exact functions independently of the location of the temperature sensors within the body. The proposed strategy is fast, robust, and reliable, which demonstrates the suitability of employing the sequential gradient-based implementation, together with the quasi-Newton updating strategy, for reconstructing transient heat transfer coefficients in the context of one- and two-dimensional non-linear heat conduction problems. Thus, the proposed method is a novel alternative strategy to other online inverse estimation procedures.

Thermo-mechanical stress analysis within a steel exhaust valve of an internal combustion engine

The valves of an internal combustion engine play an essential role in the automobiles and their surroundings significantly affect their thermo-mechanical behavior. The work aims to assess numerically the effect of the real thermo-mechanical boundary conditions on the valves by considering the actual complex surrounding. For this purpose, we have subdivided the valve into seven adequate zones. We have evaluated the average values of the transient heat transfer coefficient, the adiabatic wall temperature and the mechanical load at each subdivision are during the opening and the closing periods. A transient Finite Element Model under ANSYS APDL software is developed and simulations are carried out until reaching the steady state. The temperature distribution and the thermal stresses at each valve position is obtained and then analyzed. The main findings show that the stress intensity distribution is developed in the zones labelled stem guide port and seat local of large temperature gradients, which causes high thermal stresses responsible of cracks or thermal fatigue damage. In addition, knowing the temperature map, the thermal gradient and stress under actual conditions will surely help manufacturers to better design exhaust valve, avoid early failure and enhance the durability of valves.

Single and Double Drop Impingement on a Heated Plate CFD and Experimental Investigation

A drop impingement on a hot surface is known as an effective way for heat removal. The present work uses a VOF (volume of fluid) model to simulate the flow behavior and temperature distribution on a 50 °C heated plate while relatively cold water drop is impinging. The temperature distribution at the impingement zone is used to examine the transient heat transfer coefficients using a single drop and double drops conditions. The spreading factor is tested in both cases. A test rig is built to verify the temperature distribution and a heat balance method is introduced to find the experimental heat coefficients on the heated. The CFD solution and its flow results gives a good agreement for single drop previous results. The results for the double drop condition show a high tendency for rebound and splash of the drop leaving the central zone without water drop coverage which way causes a burn out in case of high heat fluxes. The single drop condition show a symmetrical temperature and heat transfer coefficients distribution while the double drop impingement gives lower value of coefficients with non-uniform and unsymmetrical distribution specially at the bigger drop to plate distances. The experimental average heat coefficients gives relatively low error of only 5% in case of double drop when compared to the single drop values.

Determining transient heat transfer coefficient for dropwise condensation in the presence of an air flow

It is necessary to understand humid air condensation because of its various applications, such as water harvesting. In this study, the influence of airflow on the condensation heat transfer coefficient of humid air on a horizontal surface was investigated experimentally. Airflow velocities from 1 to 15 m/s were investigated, because the condensate's shedding usually happens in this range. A relative humidity of 10–80% and subcooling temperature (Tsc) of 0–10 °C were used to achieve both single-phase and condensation regimes. To study the relationship between condensate morphology and heat transfer coefficient (HTC), transient and instantaneous heat flux measurements were required. To facilitate this, a transient inverse heat conduction method was developed to characterize the time-varying surface heat flux and associated heat transfer coefficient. A 5-fold decrease in response time was found for the transient method compared to the steady-state method. The effect of condensation parameters on the heat transfer coefficient, as well as the relationship between condensate morphology and heat transfer coefficient, is discussed. The results show that HTC for the subcooling of T = 0 °C is smaller than for other temperatures. Also, the effect of RH on condensation was investigated and higher HTC was found for higher RHs. The results clearly show that the shedding of condensate kept the average droplet size low and doubled heat transfer performance improvement.

Experimental and numerical study of flow and thermal transport in fractured rock

An experimental and numerical study was carried out to investigate matrix-fracture thermal transport in fractured rock. Two different synthetically fractured core plugs were used during the flow-through experiments while core plugs’ outer surface was maintained at two different constant temperatures. To investigate the matrix-fracture thermal transport, cold water was injected through the single fracture core plugs at different flow rates. A film type heat flux sensor was used in the fractured core plug to measure the fracture temperature and heat flux over the matrix-fracture interface. Experimental results showed that fracture temperature decreases with increasing flow rate while heat flux increases. At high flow rates, temperature difference over matrix-fracture interface increases due to large volume of water contacting the fracture at a given time. Water temperature decreased more in the case of fracture surrounded by rock matrix with low volumetric heat capacity and thermal conductivity. A numerical model calibrated using experimental values of the fracture temperature was developed to estimate thermal properties of rock matrix and to identify their importance on the matrix-fracture thermal transport. Using the calibrated numerical model, it has been shown that temperature in the fracture and fracture surface temperature are different contrary to previous studies assuming the same values for these two temperatures. Local and transient convective heat transfer coefficients were introduced with respect to local values of temperature difference and transient values of heat flux along matrix- fracture interface enabling accurate representation of matrix-fracture thermal transport.