Experimental Heat Transfer Research Articles

Heat Exchanging Grid Structures Based on Laser-Based Powder Bed Fusion: Formation Process and Boiling Heat Transfer Performance

Microchannel structures possess high efficiency and high boiling heat transfer coefficient of two-phase flow. In particular, the grid structure has the advantages of a simple pattern, large load capacity, and good surface adaptability. Employing the laser-based powder bed fusion (L-PBF) manufacturing technology, a new method of forming heat transfer grids with a controllable structure is proposed in this study. The formation principle, process, and the reasons for improvements in the boiling heat transfer performance were investigated with stainless steel materials. Laser scanning with varying scan spacings was used to prepare multiple structures with different grid widths and wall heights. On this basis, the porosity and pore morphology of the grid structures were analyzed, followed by pool boiling heat transfer experiments. The results revealed that the grid structure significantly affected the nucleate boiling behavior and increased the critical heat flux (CHF). It was found that the 0.5 mm sample exhibited optimum critical heat transfer performance, with an improvement of 10–27% compared to those of the other four samples (minimum of 63.3 W·cm−2 and maximum of 93.9 W·cm−2). In addition, for samples with a grid width greater than 0.5 mm, the boiling slightly decreased by <5%. When the grid width was further increased, the flow resistance effect and the bubble synapse generation effect tended to converge. In these cases, boiling heat transfer only occurred in a single phase along the direction of the medium wall thickness, thus failing to achieve two-phase heat transfer through bubble growth and collapse.

Numerical and experimental investigations on intense pulsed light sintering of silver nanoparticle inks for printed electronics

Intense pulsed light (IPL) sintering has the advantages of high efficiency and selective heating for silver nanoparticles (AgNPs) inks, which could be widely used in printed electronics. In this contribution, the heat transfer and diffusion mechanisms of AgNPs during IPL sintering are quantitatively investigated with a mathematic model combined transient heat transfer and molecular dynamics (MD) method and experiments. The results show that only 51.522% of the IPL radiation energy is absorbed by the AgNPs. During IPL sintering, the temperature of AgNPs rises significantly, whereas the temperature of polyamide substrate keeps almost unchanged, demonstrating that IPL is feasible for thermal-sensitive substrates. The MD modeling results show that sintering neck between AgNPs forms and grows rapidly at the beginning of sintering, then remains relative stable. Accordingly, the electrical resistivity of the AgNPs drops rapidly to a stable value. In addition, the effects of IPL sintering parameters are studied and the results show that increasing IPL energy and reducing IPL duration could increase the sintering performance of AgNPs inks. Finally, an antenna is fabricated using AgNP inks and IPL sintering technology. The experimentally measured performance of the antenna agrees well with the theoretical analysis. Our simulation and experimental results demonstrate that IPL is suitable for sintering of AgNPs inks for flexible electronics.

Direct solar-thermal conversion features of flowing photonic nanofluids

Efficient solar energy capture and flowing fluid heating are very important for solar-thermal conversion applications. In this work, through multi-scale theoretical and experimental analysis, the application features of flowing photonic nanofluids in direct solar-thermal conversion were studied. On the one hand, at the microscopic level, the process of full-spectrum sunlight capture by photonic nanofluid was investigated. Specifically, the interactions among particles in Fe3O4/TiN photonic nanofluids were analyzed in detail, as well as the mechanism of sunlight capture enhancement caused by spectral complementarity and near-field effect of nanoparticles was expounded. On the other hand, through macroscopic flow heat transfer calculations and experiments, this work studied the direct solar-thermal conversion effects of photonic nanofluids under different application conditions, including solar concentration ratio, flow rate, working temperature, and sunlight intensity. Both the experimental and theoretical results clearly illustrate that the photonic nanofluid has excellent solar-thermal conversion ability, and can be well adapted to various solar-thermal applications. This work provides a detailed method and performance parameter reference for the nanofluid selection and the working condition adjustment of direct solar-thermal conversion.

Heat transfer experiment with the prototypical Ra number of heavy metallic layer in IVR strategy

In-Vessel Retention (IVR) strategy is significant to limit molten corium in the Reactor Pressure Vessel (RPV) during severe accident scenarios. A heat transfer experiment has been established to study the thermal load on the cooled wall adjacent heavy metallic layer. The test section of the heavy metallic layer experiment is a hemisphere, and its diameter is 2.4 m; the height of simulant is set to 0.3 m. Heavy metallic layer experiment uses water as the simulant, and it uses the internal heating coils to represent the decay heat. Three independent sidewall coolant channels are used to cool the whole test section. The top boundary condition of the test section could be a cooling or insulated condition. The experimental results illustrate that the Mayinger heat transfer correlation could predict the heavy metallic layer experiment results, and it is recommended to predict the sideward heat flux. Applicating the heat transfer correlation and experimental qlocal/qmean distribution to the heavy metallic layer in core degradation situation, the heat transfer correlation calculated results demonstrate that it is unlikely for the local sideward heat flux to exceed the Critical Heat Flux (CHF). The possible top cooling boundary for the heavy metallic layer contributes to alleviating wall melting. Generally, it is unlikely that the vessel failure occurs at the cooled wall adjacent the heavy metallic layer.

Experimental study of condensation heat transfer of R134a inside the micro-fin tubes at high mass flux

The flow condensation heat transfer coefficient of R134a inside two helical micro-fin tubes with outer diameter of 6.35 mm and helical angles of 18o and 28o respectively were experimentally studied at a high mass flux ranging from 500 to 1100 kg m−2 s−1. The condensation temperatures are 35°C, 40°C and 45°C, and the vapor qualities ranges from 0.95 to 1.00 at the inlet section and from 0.00 to 0.05 at the outlet section. Experimental results show that there are annular and intermittent flow regimes inside the tube under experimental conditions, the ratio of annular flow region to the total heat transfer region is about 70%. The liquid film flow changes from rotational flow to coexistence of rotational and horizontal flows when the vapor quality is 0.475±0.08, 0.486±0.06 and 0.521±0.05, respectively and the corresponding saturation temperature is 35°C, 40°C and 45°C. Based on the experimental data, the heat transfer coefficient increases with increasing mass flux and fin helical angle, and with decreasing saturation temperature and Reynolds number of cooling water. In addition, the heat transfer coefficient of annular flow is greater than that of intermittent flow, in other words, the heat transfer coefficient increases with increasing vapor quality. The heat transfer coefficient obtained at high mass flux was compared with the calculated data of some published correlations used at low mass flux. It was found that most correlations underestimate the experimental heat transfer coefficient inside the micro-fin tube, and these prediction deviation increases with increasing mass flux and fin helical angle. By introducing some dimensionless numbers as well as considering the flow mechanism of liquid film inside the tube, two heat transfer coefficient correlations were proposed based on the experimental data. The ratio of liquid film thickness to fin height was used to characterize whether the liquid film flow is rotational flow or coexistence of rotational and horizontal flows in the proposed correlation, which shows a better prediction accuracy with a mean deviation of less than 7.3%.

Experimental heat transfer results and flow visualization of vertical upflow boiling in Earth gravity with subcooled inlet conditions – In preparation for experiments onboard the International Space Station

Since 2012, researchers at the Purdue University Boiling and Two-Phase Flow Laboratory (PU-BTPFL) and NASA Glenn Research Center have been collaborating on a long-term effort to study flow boiling and condensation in microgravity. The ultimate goal has been to develop the Flow Boiling and Condensation Experiment (FBCE) for the International Space Station (ISS). Based on the findings from prior flow boiling experiments both at different orientations in Earth gravity and onboard parabolic flights simulating short durations of microgravity, a final refined experiment design, construction, and operating procedure have been arrived at for long-duration microgravity flow boiling experiments onboard the ISS. This study investigates flow boiling of n-Perfluorohexane with subcooled inlet in a rectangular channel of dimensions 114.6 mm heated length, 2.5 mm width, and 5 mm height. These pre-launch experiments (Mission Sequence Testing) were conducted in vertical upflow orientation in Earth gravity using the same experimental rig that was launched to the ISS in August 2021. The various operating parameters varied are heating configuration (single- and double-sided), mass velocity (180 – 3200 kg/m2s), inlet subcooling (+0 – 32 °C, encompassing both highly subcooled and near-saturated inlet conditions), and inlet pressure (119 – 191 kPa). High-speed video flow visualization images are presented to explain the two-phase interfacial physics within the channel's heated section. Heat transfer results in terms of flow boiling curves, streamwise profiles of wall temperature and heat transfer coefficient, and averaged heat transfer coefficients are analyzed and parametric effects elucidated. Severe temporal thermodynamic equilibrium is observed for near-saturated inlet at very low velocities. Nucleate boiling degradation starts at larger heat fluxes for single-sided heating than double- sided at low mass velocities with highly subcooled inlet, and conversely at high mass velocities with near-saturated inlet. Nucleate boiling degradation can be delayed to higher heat fluxes by highly subcooling the inlet and increasing mass velocity. The entire local heat transfer coefficient profiles are degraded at higher heat fluxes for near-saturated inlet, but only the downstream part for highly subcooled inlet. This study also confirmed reliability of the upcoming ISS experimental data for subcooled inlet conditions and the collected Earth-gravity data will be used for comparison against the ISS data.

Turbulent Flow in Forced Convection Heat Transfer - Numerical Validation

Forced convective heat transfer of airflow through circular pipe with constant heat input and different free stream velocities is numerically validated. The significance of the present work is that the suction flow has been employed in the forced convection set up domain kept in the wind tunnel. From first law of thermodynamics and applying the energy balance equation, experimental heat transfer coefficient is determined. Further correlations are used to validate the experimental results. Although correlations provide reasonable estimates from the point of feasibility and accuracy, computational methods are used to estimate the convective heat transfer coefficient. Hence in this paper experimental, theoretical and computational analysis is carried out. The results reveal that the numerical validation is an effective tool from the point of feasibility and accuracy to determine the convective heat transfer coefficient.

Experimental Flow Boiling Study of R245a at High Reduced Pressures in a Large Diameter Horizontal Tube

Evaporators used in organic Rankine cycles (ORC) are designed using existing flow boiling correlations that are mainly based on HVAC&R data. However, the ORC evaporators employed in the industry typically have larger diameters and operational conditions at higher reduced pressures compared to the HVAC&R applications. The present study presents the results of flow boiling heat transfer experiments in operational conditions that are representative for an industrial waste heat recovery low-temperature ORC’s evaporator tube, by being performed at high reduced pressures and in a large-diameter horizontal tube with R245fa as working fluid. The measurements are performed within a range of mass flux, saturation temperature and heat flux at 83–283 kg/m2s, 85–120 °C (8.9–19.2 bar) and 17–29 kW/m2, respectively. Test section is a round and plain horizontal carbon steel tube with 21 mm I.D., 2.5 m length. 513 local two-phase heat transfer coefficients are recorded. The experimental results are compared with each other to reveal heat transfer coefficient trends with respect to varying experimental conditions. Four distinctive heat transfer zones are observed, namely, the nucleate boiling dominant (NBD) zone, weakening nucleate boiling dominance (WNBD) zone, flow boiling zone (FBZ) and the dry-out zone (DOZ). Heat transfer coefficient vs vapor quality trends partly resembled CO2 flow boiling results reported in the literature. Two flow boiling correlations moderately predicted the data.

Counterflow HE analysis of Cu and SiO2 nanofluids in the developing flow region

AbstractThree concentrations of 0.2, 0.6, and 1.0 vol.% Copper/25 nm and silica/22 nm nanofluids are prepared in a base liquid glycerol–water mixture of 30:70 ratio by volume (GW70). The thermophysical properties of Cu and SiO2 nanofluids are determined with a TPS500S hot disc thermal analyzer and Brookfield viscometer in the temperature range of 20–80°C. The maximum enhancement in Cu and SiO2 nanofluid viscosity (63.4%, 35.7%), thermal conductivity (100.4%, 71.3%), and density (7.5%, 1.5%) while specific heat (7.8%, 2.3%) determined for 1.0% concentration at 80°C compared to base liquid GW70. Heat transfer experiments are conducted in a short‐length double pipe heat exchanger. The flow rates resulted in the lamifnar entry length region. A maximum enhancement in the overall heat transfer coefficient (HTC; 25.0%, 19.7%) and convective HTC (46.2%, 34.8%), respectively for Cu and SiO2 nanofluids is estimated at 1.0% concentration compared to base liquid at a bulk temperature of 35°C.

A Monte Carlo approach to evaluate the local measurement uncertainty in transient heat transfer experiments using liquid crystal thermography

This work presents an improved estimation of the local measurement uncertainty of transient heat transfer experiments using Thermochromic Liquid Crystals (TLC). Additional signal analysis steps and a Monte Carlo (MC) approach are added to the evaluation routine. As a result, the local uncertainty in the indication time of the maximum green intensity is determined. All other input quantities for the heat transfer coefficient calculation are provided with their characteristic distribution function introducing additional uncertainties. As a final result, lower and upper boundaries for a confidence interval with a user defined confidence level for the heat transfer coefficient are calculated locally for single pixels. The new method is applied to own measurement data from a transient TLC experiment. The local measurement uncertainty is compared to the traditional approach based on error propagation. Hence, the advantages of taking the non-linearity and the local uncertainty in the peak detection into consideration are shown.

Design of thermal cloak and concentrator with interconnected structure

Heat flux manipulation based on thermal metamaterials under diffusive heat conduction has been extensively studied due to their extraordinary potential in energy utilization. However, the requirement of extreme material parameters or even infinite singularity may significantly limit their practical applications. In this work, we propose a new method for uniformly designing thermal cloak and concentrator with interconnected structure. The condition for realizing the undisturbed temperature distribution of the background medium in thermal cloak is strictly deduced. Through adjusting two geometric parameters of the interconnected structure, we analytically demonstrate that the thermal cloak and concentrator are realizable, where the heat flux can be shielded or concentrated in the target region. The desired thermal functionalities are confirmed both by the finite element simulations and heat transfer experiments. Furthermore, the results of interconnected structure are in excellent agreement with that of conventional thermal devices, and demonstrate that the interconnected structure provides an efficient method for manipulating heat flux over a very wide range (from cloaking to concentrating) despite having a simple composition and structural characteristics. The feasibility of interconnected structure can enhance the selection of bulk natural materials in addition to easy manufacturability for the heat flux manipulation, which might be extended to other disciplines, such as electromagnetics and acoustics.

Experimental study of a dual condenser (EVACON) with concentric helicoids in use with an absorption heat transformer

The purpose of the present work is the experimental characterization of an innovative dual condenser (performing two unitary operations under a single shell) with nested helicoids and obtain the experimental overall heat coefficient that can be used in the future for designers. The device was designed, characterized, and experimentally tested, ready to be adapted into an absorption heat transformer on a pilot scale of 5000 W of heat capacity. The internal geometry of the condenser consists of 7 concentric coils, nested in a rigid shell and connected in parallel through a manifold that distributes the service fluid into the coils. The steam from the evaporator condenses on the outside of the tubes creating a uniform falling film. The construction material is 304 stainless in order to secure a prolonged service life. The experimental evaluation consisted of 24 experimental tests that were carried out using water as a working and service fluid at 4 different temperatures and 6 different feed flow rates from the source. The study consisted of determining the heat capacity rate of the condenser as well as its heat transfer coefficient. The heat capacity rate was calculated using an energy balance in the dual device. We used correlations to helicoidally geometry to determine the overall heat transfer and convective heat coefficients. The sizing method was validated taking into account the theoretical and experimental overall heat transfer coefficient of 678 W/m2 K.The results show the maximum heat capacity of the condenser at 4179 W. The overall heat transfer coefficient varies between 441 and 614 W/m2°C, while the convective heat transfer coefficient for condensation (film side) ranges between 1288 and 1615 W/m2°C. These values are higher than those previously reported for similar coil heat exchangers in similar operation conditions. These results let us know that the condenser will have a good performance when adapted into a heat absorption machine.

Flow instability and heat transfer enhancement of unsteady convection in a step channel

The formation of flow instability and heat transfer enhancement of unsteady convection is studied for backward-facing step flow, forward-facing step flow and combined step flow. The governing equations are solved by a program of FORTRAN code. The numerical method is validated by PIV and heat transfer experiments. The effects and the affecting factors of flow instability are investigated. The results show that the flow instability caused by the reattachment flow has a positive effect on heat transfer. In combined step flow, the formation of this flow instability is mainly affected by the bottom wall length and the critical Reynolds number. The minimum critical Reynolds number of the transition from steady to unsteady case is between 200 and 250. The fundamental flow pattern results show that 7 typical flow characters appear in different step channel. A new concept named the vortex length ratio (LRv) is proposed to distinguish the vortex shape. The vortex with the smaller LRv is more effective in heat transfer enhancement mainly because of the increase of vortex attack angle and vortex rotating speed.

Experimental and numerical evaluation of the effect of micro-aeration on the thermal properties of chocolate.

Thermal properties, such as thermal conductivity, specific heat capacity and latent heat, influence the melting and solidification of chocolate. The accurate prediction of these properties for micro-aerated chocolate products with varying levels of porosity ranging from 0% to 15% is beneficial for understanding and control of heat transfer mechanisms during chocolate manufacturing and food oral processing. The former process is important for the final quality of chocolate and the latter is associated with sensorial attributes, such as grittiness, melting time and flavour. This study proposes a novel multiscale finite element model to accurately predict the temporal and spatial evolution of temperature across chocolate samples. The model is evaluated via heat transfer experiments at temperatures varying from 16 °C to 45 °C. Both experimental and numerical results suggest that the rate of heat transfer within the micro-aerated chocolate is reduced by 7% when the 15% micro-aerated chocolate is compared to its solid counterpart. More specifically, on average, the thermal conductivity decreased by 20% and specific heat capacity increased by 10% for 15% micro-aeration, suggesting that micro-pores act as thermal barriers to heat flow. The latter trend is unexpected for porous materials and thus the presence of a third phase at the pore's interface is proposed which might store thermal energy leading to a delayed release to the chocolate system. The developed multiscale numerical model provides a design tool to create pore structures in chocolate with optimum melting or solidifying response.

Theoretical and Experimental Laminar Fluid Flow and Heat Transfer of Circular and Different Corrugation Heat Sink

Theoretical and Experimental Laminar Fluid Flow and Heat Transfer of Circular and Different Corrugation Heat Sink

Experimental and simulation study on heat and mass transfer characteristics in direct-contact total heat exchanger for flue gas heat recovery

Direct-contact total heat exchange with liquid desiccant produces enormous gains in both sensible and latent heat recovery of low-grade flue gas. Previous studies related to that has tended to focus on the system performance rather than the heat and mass transfer characteristics, which is equally essential to their design, simulation and operation. This paper establishes an experimental system to investigate the heat and mass transfer characteristics in direct-contact total heat exchanger for flue gas total heat recovery. Ceramic structured packing is adopted considering its corrosion and high-temperature resistances; calcium chloride aqueous solution is employed as the coolant and absorbent for its low cost and high latent heat recovery capability. Based on those, the effects of flue gas and solution inlet parameters on the performance indices, including sensible and latent heat recovery capacities, latent heat rate and total heat recovery rate, are tested and analyzed. Coupled heat and mass transfer models between flue gas and solution in this process are developed, which feature the enthalpy of dilution. By combining the models and experimental data, heat and mass transfer coefficients are calculated and fitted. In addition, the effect of packing height on total heat recovery capacity and ratio is investigated through simulations. The results show that the total heat recovery rate of 74.7% can be achieved at packing height to 1.5 m when liquid–gas ratio is 5.172.

Gasdynamics Aspects of the Heat Transfer Experiment with the UHTC Surface in Under-Expanded Dissociated Air Jet

Gasdynamics Aspects of the Heat Transfer Experiment with the UHTC Surface in Under-Expanded Dissociated Air Jet

Thermal and hydraulic performance of a compact precooler with mini-tube bundles for aero-engine

It is significant to evaluate whether the classical correlations based on conventional tube size (≥10 mm) is suitable for predicting the flow and heat transfers characteristics of the precooler composed of mini-tube bundles (around 1 mm). In this work, a full-scale heat transfer experiment of precooler is carried out, and the high-temperature incoming air (686 K) in the experiment was produced by burning the mixture of air and alcohol. The numerical simulation of a unit precooler is also conducted with Computational Fluid Dynamics. The classical correlations for the design of heat exchanger are evaluated by both experiments and simulations. The results indicate that the prediction error of classical correlations of flow resistance is less than 10% for both the flow inside the mini-tube and across mini-tube bundles. The classical correlation of heat transfer inside tube can meet the engineering requirements. However, the maximum deviation of Nusselt number prediction for the outside flow across tube bundles reaches 32.1%. A new correlation for calculating heat transfer across a compact precooler composed of staggered mini-tube bundles with a prediction error below 5% is finally established based on the experimental and numerical results.

Conduction and Inertia Correction for Transient Thermocouple Measurements. Part II: Experimental Validation and Application

Thermocouples are often used for temperature measurements. Under transient conditions, measurement errors can occur due to capacitive inertia and heat conduction along the stem of the thermocouples. To correct such errors, a method is presented in Part I [1] of this paper, which uses a simplified analytical approach and a numerical solution. In the present work, this method is applied to temperature measurements. Several experiments with different thermocouple designs were performed to investigate different conditions such as installation depth, thermocouple type and transient temperature rises. In all cases, two thermocouples were placed so that they are exposed to the same fluid temperature. They are installed with short or long immersion length, respectively. It is shown that only the short thermocouple experiences a thermal conduction error, but both are subject to thermal inertia. The importance of compensating for these effects is shown by quantifying the errors in a typical heat transfer experiment when they are neglected. It is shown, which parameters are necessary for a re-calculation of fluid temperatures when two thermocouples are present at the same measuring position. Furthermore, a simplified method is described, which can be applied if the instrumentation of only one thermocouple is possible.

Experimental study on heat transfer in a model of submerged combustion vaporizer

• Heat transfer experiment of submerged combustion vaporizer (SCV) model is conducted. • The shell-side, tube-side and overall heat transfer coefficients are obtained. • The average of shell-side heat transfer coefficient is about 300 W/(m 2 ·°C). • Two simple empirical correlations are developed for the overall tube-side heat transfer. The submerged combustion vaporizer (SCV) is widely used in the peak shaving of liquefied natural gas (LNG) supply. It is believed that SCV will have a great application prospect with the implementation of the peak-shaving policy of natural gas in China. SCV injects the flue gas after combustion directly into the water bath from its bottom, causing gas disturbance and two-phase flow in the water bath. The shell-side flow and heat transfer become unclear because of the gas disturbance. There is a lack of relevant theoretical basis in this regard. In order to master the operation mechanism of SCV, an experimental research is carried out on a SCV model which has the same structure and function as the actual small-size SCV. The shell-side, tube-side and overall heat transfer coefficients are obtained, and the influences of operation parameters on the heat transfer are investigated. The results indicate that in the range of the experimental parameters, the shell-side heat transfer coefficient is almost not affected by the frontal gas velocity and Reynolds number of the flue gas at orifices, and the average value is about 300 W/(m 2 ·°C). Meanwhile, the higher the flow rate and pressure of the tube-side medium, the greater the overall heat transfer coefficient. In addition, it is observed that if the gas disturbance stops, the heat transfer deterioration will occur. Finally, two simple empirical correlations are developed for the overall tube-side heat transfer. This study provides support for the design and application of SCV.