Reduction In Heat Transfer Coefficient Research Articles

Numerical Investigation of Melting in Pressurized Water Reactor Fuel Rod Considering Operational Parameters of the Core

The meltdown of a fuel rod is a severe accident resulting from the overheating of the reactor core. In the present study, a numerical investigation of this process, focusing on the loss of coolant has been conducted. The objective of this study is to conduct a numerical simulation of transient heat conduction and melting at various points within a typical pressurized water reactor fuel rod. In this analysis, heat conduction in the radial direction of a fuel rod, including the UO2 fuel pellet, the gap, and the zircaloy cladding, is investigated. The FRAPCON steady-state code is employed to calculate the operational parameters of the fuel rod. The calculated parameters, such as coolant and fuel temperatures, fission gas fraction, gap heat transfer coefficient, and burnup, are utilized to evaluate and compare the melting phenomena at different time intervals. In the investigation of the phase change in various parts of the pellet and fuel rod, the explicit finite difference (FD) method is utilized with enthalpy instead of temperature-dependent equations. Finally, the temperature history, phase change, and melting map at different points along the radial and axial directions of the fuel rod during coolant loss and heat transfer coefficient reduction are evaluated based on various operating parameters of the core. To enhance the quality of the results, an uncertainty analysis of effective parameters is conducted. According to this analysis, the heat transfer coefficient of the coolant under accident conditions (0.2 ± 5% kWm−2K−1) and the thermal conductivity of the fuel have the most significant impact on the temperature history and melting process. Highlights include the following: 1. The meltdown of a nuclear fuel rod is analyzed under a loss-of-coolant accident. 2. The enthalpy formula is discretized by the explicit FD numerical method. 3. Effective parameters in melting, such as coolant temperature, burnup, and gap heat transfer coefficient, are obtained by FRAPCON. 4. The temperature history, phase changes, and melting map of various radial points within the fuel pellet and cladding along the axial direction of the fuel rod are determined.

Performance analysis of irradiated thermo-mechanical coupled petal-shaped fuel rod under different power and heat transfer conditions

As a new type of fuel rod, petal-shaped fuel rod (PSFR) has the advantages of lowering the core temperature and increasing the power density, and thus has a broad application prospect. Based on the thermal elastic–plastic theory, this paper establishes an analytical model for irradiated thermal–mechanical coupling performance of PSFR by user subroutine. Through numerical simulations of the full-cycle operation of PSFR under different power and heat transfer conditions, the performance parameters of fuel rods, such as fuel rod temperature and stress, are obtained, and their spatial distribution and the variation pattern with the operation time are analyzed. The results show that increasing line power leads to a simultaneous increase in fuel rod temperature, stress, maximum deformation, and creep. However, an appropriate reduction in heat transfer coefficient decreases the cladding stresses. The above research results will provide an essential theoretical basis for determining new PSFR’s deformation and operation risk.

Flow thermohydraulic characterization of open diverging microchannel heat sink for high heat flux dissipation

The future fusion reactor puts forward higher demand for the heat dissipation capability of the divertor, which requires to withstand ultra-high heat flux of up to 20 MW/m2. Our previous study demonstrated that microchannel cooling technology is a promising strategy to remove high heat loads on the target plate of the divertor. However, the high pressure drop of the straight rectangular microchannel limits its practical implementation. In this study, we propose an open diverging microchannel heat sink (ODMHS) to reduce the pressure drop under high heat flux and high flow rate conditions. The effects of the width and height of expanding channel on the heat transfer performance and pressure drop were first studied numerically. The results indicated that expanding channel width resulted in a reduction of heat transfer coefficient, whereas the expansion of the channel height led to the enhancement of heat transfer performance. Regardless of the divergence of channel width or height, the enlargement of the channel dimension can effectively reduce the pumping power. Subsequently, experimental tests were carried out in ODMHS with a width ratio and a height ratio of 1.33 and 1.30, respectively. The results demonstrated that the ODMHS can withstand heat flux of up to 15.6 MW/m2 at 3 L/min. This study provides important insights for high heat flux dissipation on the target plate of the divertor by rational design of microchannel structures.

Experimental investigation of condensation heat transfer of zeotropic refrigerant/oil mixtures in plate heat exchanger

Plate heat exchanger (PHE) has been wildly used as the condenser of the organic Rankine cycle (ORC). As a potential zeotropic working fluid of ORC system, it is of great significance to study the condensation heat transfer of the mixture of R245fa/R141b and lubricating oil in PHE for the development of ORC system. In this paper, an experimental study on the condensation heat transfer coefficient (HTC) of a mixture of R245fa/R141b (0.5/0.5 by mass) and polyester oil (POE) in a plate heat exchanger (PHE) has been carried out. In addition, the results were compared with those of R245fa and R141b. The parameters in the experiment range from 0 to 0.9 for vapor quality, 0%–4% for oil concentration and 20 to 60 (kg·m−2·s−1) for mass flux. The results showed a 20% reduction in HTC for R245fa/R141b with 4% oil concentration compared to oil-free refrigerant. In addition, the HTC of R245fa/R141b-POE mixture was between that of R245fa/POE and R141b/POE. The oil enhancement factor of different refrigerant/POE mixtures was less than 1.0, which could be well predicted by the Eckels correlation with a maximum mean absolute deviation (MAD) of 13.4%. The product of the predicted enhance factor and HTC predicted by the Jung et al. correlation agreed well with the tested HTC of different refrigerant/POE mixtures in the PHE. The maximum MAD of different refrigerant/POE mixtures was 20.6%, and 96.3% of predicted results were within ±30% deviation. A new correlation with a maximum MAD of 15% was proposed, which predicted the condensed HTC of refrigerant/POE mixture by using its physical properties.

Pool boiling heat transfer characteristics of R-1233zd(E) on smooth tube subject to POEC-220 lubricant oil

Pool boiling heat transfer characteristics of low pressure and low GWP refrigerant R-1233zd(E) alternative to R-123 and R-245fa on a smooth copper tube are investigated with the influence of POE (polyolester)-lubricant oil POEC-220 (220 cSt). The experiments were conducted at a saturation temperature of 10°C, and 26.7°C in 10−90 kWm−2 heat flux range. The mass concentration of POEC-220 oil varied from 1−10%. The obtained results exhibit that the addition of POE lubricant to the pure low-pressure refrigerant R-1233zd(E) degraded the heat transfer performance. The average degradation of heat transfer coefficients (HTCs) for mass concentration of oil 1%, 3%, 5%, and 10% are 9.40%, 7.16%, 15.31%, and 42.87% respectively when compared to that of pure refrigerant. It is found that the nucleation sites are especially reduced at the bottom part of the test tube. Yet, either Augmentation or reduction of HTC with the presence of lubricant oil is mainly attributed to the absolute pressure (low/medium/high) of the working refrigerant/fluid/system/evaporator, the mass concentration of oil, boiling surface, and applied heat flux. However, augmentation in the HTC with less oil addition (e.g., less than 5%) can be seen for absolute pressures between 150 and 700 kPa.

Investigation on the effect of the heating surface temperature of 1st evaporator on sucrose loss and the degradation of sugarcane juice constituents

To enable more energy efficient sugar production from sugarcane, higher process temperatures in the 1st evaporator is typically utilized, as this allows higher temperature bleed vapours to be used rather than process steam for other heating duties in the factory. This invariably increases juice degradation and the temperature difference (ΔT) between the heated surface and juice can be even higher where process steam used for heating is superheated. This study investigated the effect of high surface temperatures (139–168 °C) and residence times (0–60 min) on both factory and synthetic juice degradation using a specially designed laboratory evaporator rig. The results show that ΔT is inversely proportional to the heat transfer coefficient. At up to 148 °C and short residence time, there were minor differences in the juice degradation profile. However, at higher temperatures there were significant sucrose loss (≤2.5%), amino acid degradation, lignin (associated with fibre in juice) depolymerization to hydroxycinnamic acids, isomerization of trans -to cis -aconitic acid, and the formation of alcohols. Strategies to minimize juice degradation in evaporators are highlighted. • Rate of sucrose degradation in sugarcane juice increased with Heating Surface Temperatures (HSTs) (<2.5% at 168 °C). • The rate of degradation of sugarcane juice constituents increased with increasing HSTs. • Sugarcane juice degradation caused increased colour and higher pH drop in the juice. • Higher temperature differentials in evaporators cause reduced heat transfer coefficients. • Sugarcane juice contains catalytic impurities (i.e., minerals) that significantly increase sucrose degradation.

Study of heat transfer to supercritical pressure water across a wide range of parameters in pulse heating experiments

• Controlled pulse heating of supercritical pressure water was performed. • Unexpected response to the short-term release of high-power heat was revealed. • It is consistent with the character of the density change over supercritical isobars. • Reduced heat transfer coefficient exhibits a minimum at pseudocritical temperature. • We attribute this minimum to the suppression of fluctuations in short-term experiment. From experimental data on pulse heating of a thin wire probe in water pre-compressed to supercritical pressures at room temperature, the instantaneous heat transfer coefficient has been calculated over a wide temperature range from 250 °C to 700 °C. The pressure, being the parameter of the experiment, has been varied from the near-supercritical value (22.1 MPa) to 100 MPa. The heat flux density across the probe surface reached 10 MW/m 2 at a characteristic heating time of 20 ms. Under conditions of powerful heat release and small temporal and spatial scales, heat transfer is carried out mainly due to the heat conduction mode, which is the most physically determined mechanism of heat transfer. It has been revealed that the reduced heat transfer coefficient exhibits a minimum in the region of pseudocritical temperatures; this minimum becomes deeper, as the pressure value approaches the critical one. The obtained data can be useful in the design of heat exchange devices, the operation of which may include powerful local heat release, as well as in the simulation of emergencies associated with a sharp jump in reactivity in nuclear power units.

Heat transfer characteristic of a fully cooled turbine vane

To evaluate the heat transfer performance of a fully cooled turbine vane, an experiment was conducted in the low-speed wind tunnel using transient liquid crystal technology. The test vane was fabricated with 18 rows of cylindrical holes, which were supplied by two plenums. In these experiments, three mass flow ratios of 5.5%, 8.4%, and 12.5%, three turbulence intensities of 2%, 9%, and 15%, three Reynolds numbers of 300000, 400000, and 500000, two density ratios of 1.0 and 1.5 were tested. The results show that the heat transfer coefficient in the leading edge is much higher than the pressure and suction side due to the direct impact of the mainstream and the severe mainstream and injection mixing. The increased mass flow ratio significantly increases the heat transfer coefficient over the entire vane surface by 3.6%–71.3%, and the elevated turbulence intensity enhances the heat transfer coefficient by 5%–45% on the suction side and the rear half of the pressure side. The higher Reynolds number causes the thinner boundary layer and the higher velocity near the wall, which results in the Nusselt number increases by 52% in the case of increasing the Reynolds number from 300000 to 500000. Additionally, the heavier coolant leads to a slightly reduced heat transfer coefficient because of the lower injection penetration.

Two-Dimensional Flow Boiling Characteristics With Wettability Surface in Microgap Heat Sink and Heat Transfer Prediction Using Artificial Neural Network

Abstract As technology becomes increasingly miniaturized, thermal management becomes challenging to keep devices away from overheating due to extremely localized heat dissipation. Two-phase cooling or flow boiling in microspaces utilizes the highly efficient thermal energy transport of phase change from liquid to vapor. However, the excessive consumption of liquid-phase by highly localized heat source causes the two-phase flow maldistribution, leading to a significantly reduced heat transfer coefficient, high-pressure loss, and limited flow rate. In this study, flow boiling in a two-dimensional (2D) microgap heat sink with a hydrophilic coating is investigated with bubble morphology, heat transfer, and pressure drop for conventional (nonhydrophilic) and hydrophilic heat sinks. The experiments are carried out on a stainless steel (SS) plate, having a microgap depth of 170 μm using de-ionized (DI) water at room temperature. Two different hydrophilic surfaces (partial and full channel shape) are fabricated on the heated surface to compare the thermal performance with the conventional surface. Vapor films and slugs are flushed quickly on the hydrophilic surfaces, resulting in heat transfer enhancement on the hydrophilic heat sink compared to the conventional heat sink. The channel hydrophilic heat sink shows better cooling performance and pressure stability as it provides a smooth route for the incoming water to cool the hot spot. Moreover, the artificial neural network (ANN) prediction of heat transfer coefficient shows a good agreement with the experimental results as data fit within ±5% average error.

Forced Convective Boiling in a Vertical Annular Test Section With Seawater Coolant

Abstract In case of some nuclear reactors, seawater is used as an emergency resource to remove the decay heat from the reactor core. This study aims to improve the understanding of boiling heat transfer with seawater coolants. Under the boiling conditions with seawater, the mass transfer of the dissolved impurities to the heated surface is expected to significantly impact the heat transfer characteristics. The focus of this experimental work is to measure the differences of the heat transfer performance between seawater and tapwater with electrically heated cylindrical section in a vertical annulus. High speed visualization is performed to quantify and characterize the bubble dynamics parameters. The experimental results indicate an enhanced heat transfer coefficient with seawater in the initial transient under saturated boiling followed by an asymptotic reduction to values similar to tapwater. Under subcooled boiling, a consistent reduced heat transfer coefficient was observed in seawater for a range of heat fluxes. The high speed visualization of subcooled boiling showed fewer and smaller bubbles nucleating off the heat transfer surface in seawater indicating a lower evaporative flux as compared to tapwater.

Effect of Flow Bypass on the Performance of a Shrouded Longitudinal Fin Array.(Dept.M)

Theoretical and experimental studies were carried out 10 investigate the effects of duct velocity, sin density and tip-to-shroud clearance on the flow bypass and its impact on the pressure drop across a longitudinal aluminum fin array and its thermal performance. The clearance was varied parametrically, stating with the fully shrouded case and variations of the channel height giving partially shrouded configuration of different clearance ratios were also carried out. The slow bypass was found to increase with increasing sin density and insensitive to the air slow rate. That effect of fin density decreased as the clearance increased. The calculated total pressure was greatly affected by fin density. For fully-shrouded sin array, with H/S equals to 8 and 12.72, the pressure drop increased by a factor of 4.3 and 20 of that with Hf/S equals to 3.4, respectively. The total pressure drop and the average convective heat transfer coefficients corresponding to the fully and partially shrouded fin array of Hf /S=3.4 were compared. Going from fully to partially shrouded one of the largest clearance ratio (C/Hf =0.89), the total pressure drop reduced by about 50%. For clearance ratios equal to 0.36, 0.56, and 0.89, the average heal transfer coefficients were reduced by about 12, 17, and 30 percent of those for the fully shrouded configuration at ReD of about 3x103. That percentage reduction in heat transfer coefficients decreased with the increase of air flow rate.

Modelling and simulation of ultrasonic descaling in heat exchanger

Heat exchanger is widely used in petrochemical industry, long cycle operation will lead to the generation of scaling and the reduction of heat transfer coefficient, but the traditional scale removal method is not convenient to operate with high cost. In this paper, a kind of ultrasonic descaling technology is designed, and the propagation model of ultrasonic wave in heat exchanger is established by using B-P neural network, including the propagation model of ultrasonic load on tube plate and the propagation model of ultrasonic load in medium. Based on the influence of two different ultrasonic loading modes, the propagation mechanism of ultrasonic load on the tube plate of heat exchanger was obtained by simulation research. Finally, the propagation mechanism model of ultrasonic wave in heat exchanger is established successfully through experimental verification.

Thermal characteristics of a flat plate solar collector: Influence of air mass flow rate and correlation analysis among process parameters

Present research focuses on the analysis of temperature profile, heat transfer characteristics and thermal efficiency of a flat plate solar air collector (FPSAC) at different air mass flow rates. Further, the interrelationship among several variables was also investigated using multivariate analysis. Experiments were performed with FPSAC at non-load conditions in the outdoor environment at natural convection (NC) and forced convection (FC) conditions. Air with mass flow rate (ṁ) of 0.01, 0.015 and 0.02 kg/s exhibited 30.73%, 57.88% and 96.40% enhanced convective heat transfer coefficient (hc,p-a) than 0.006 kg/s. Moreover, increase of ṁ from 0.006 kg/s to 0.02 kg/s augmented the thermal efficiency (ηth) from 22.53 to 32.3%. With regard to 0.006 kg/s, air mass flow rate of 0.02 kg/s produced 8.6% and 4.3% reduced heat transfer coefficient from absorber plate to glass due to convection (hc,p-g) and radiation (hr,p-g). Pearson correlation analysis indicated a strong positive effect of solar intensity on the temperature of absorber plate, glass cover and outlet air of collector, but a weak correlation with collector inlet air temperature, useful heat gain, ηth, and hc,p-a. Further, a positive correlation of solar intensity with hc,p-g and hr,p-g was also noticed. Principal component analysis enabled the visualization of association among solar intensity, temperature of different components of collector, heat transfer coefficients, thermal efficiency and time of the day. The analysis indicated that useful heat gain, heat transfer coefficient of air and thermal efficiency of the collector were not highly influenced by solar intensity.

Heataerodynamic efficiency of small size heat transfer surfaces for cooling thermally loaded electronic components

Heat transfer surfaces are widely used to ensure normal thermal conditions of heat-generating elements in radar transmit/receive modules, computing systems, power electronics, etc. The authors offer the first ever generalization of a large experimental data set on the thermal and aerodynamic efficiency of small size flat-based heat transfer surfaces (heat sinks) of various types. The paper presents a comparative analysis on the efficiency of surfaces with different types of finning (plate-fin, cross-cut, needle-pin and mesh-wire surfaces) under forced air convection. The following efficiency criteria were chosen: the surface base superheat above the ambient temperature and the complex parameter, which takes into account geometric and heat transfer characteristics of the surfaces. This complex parameter is the product of the reduced heat transfer coefficient and the finning ratio. The results on comparing the heat transfer efficiency of the surfaces according to these two criteria are presented as graphs and histograms. It is shown that the aerothermodynamic efficiency of the surface with cross-cut finning is higher than that of the standard plate and needle-pin surfaces by 1.15–1.65 and 1.8–2.0 times, respectively, and by 3.0–3.2 times compared to the mesh-wire surface. The aerothermodynamic efficiency increases due to the appearance of separated flows on the cut fin segments, a decrease in the thickness of the boundary layer on these segments, and an increase in the air flow turbulence. The developed heat transfer surfaces can be widely used in combined air cooling systems for the modern element base with an increased heat generation.

A 2D numerical study on the condensation characteristics of three non-azeotropic binary hydrocarbon vapor mixtures on a vertical plate

To improve the transportation efficiency and reduce the supply cost, the liquefaction becomes an important technology to store and transport the natural gas. During the liquefaction, the various components (e.g. propane, ethane, methane etc.) undergo fractional condensation phenomenon due to their different boiling points. This means that when one component condenses, others play a role of non-condensable gas (NCG). In order to reveal the influence mechanism of NCG on this condensation process, a numerical method was employed in this paper to study the condensation characteristics of three non-azeotropic binary hydrocarbon vapor mixtures, namely the propane/methane (80%–95%), ethane/methane (65%–85%) and methane/nitrogen (2%–13%) mixtures, on a vertical plate. The model was proposed based on the diffusion layer model, and the finite volume method was used to solve the governing equations. A user defined function was developed by cell iterative method to obtain the source terms in the condensation process. The numerical results show that the gas phase boundary layer formed by the NCG becomes the main resistance to the reduction of heat transfer coefficient. And for the above three mixtures, there is a negative correlation between the NCG concentration and the heat transfer coefficient. Meanwhile, the results show a good agreement with the experimental data, meaning that the proposed model is reliable. Three mixtures within same non-condensable mole fraction of 20% were also investigated, indicating that the mixtures with a higher binary hydrocarbon molecular ratio have a lower heat transfer coefficient. As a result, the presence of the lighter NCG contributes to a thicker boundary layer.

FLOW BOILING HEAT TRANSFER CHARACTERISTICS OF TITANIUM OXIDE/WATER NANOFLUID (TIO2/DI WATER) IN AN ANNULAR HEAT EXCHANGER

A range of experiments was conducted to measure the heat transfer characteristics of titanium oxide/deionized water nanofluid (NF) inside a steel-made Pyrex annular system. A set of experiments was designed and performed at inlet temperature (IT) of the NF (333 K-363 K), the applied heat flux (AHF) (4.98 kW/m2 to 112 kW/m2), 1988 &lt; Re &lt; 13,588 and dispersion concentration of wt.%=0.05 to wt.%=0.15) on the average heat transfer coefficient (HTC) and boiling section’s average pressure drop (PD). It was demonstrated that the increase in the volume flow and the AHF can increase the HTC while increasing the weight concentration of the NF, initially increased the HTC such that the maximum enhancement in the HTC was 35.7% at wt.%=0.15 and Re=13500, however, over the time, the HTC of the NF decreased. The reduction in HTC was attributed to the formation of continual sedimentation on the boiling surface after 1000 minutes of the operation. The IT of the NF slightly increased the HTC, which was due to the enhancement in the thermal and physical properties such as thermal conductivity. The maximum enhancement in HTC due to increase of the IT from 333 K to 363 K was 4.2% at wt.%=0.15 and Re=13500. The bubble formation was also found to be a strong function of the applied HF such that with increasing the HF, the rate of the bubble formation increased, which was also the reason behind the augmentation in the HTC at larger AHFs. Also, the PD was augmented due to the increase in the velocity and flow and also weight concentration of NF. The highest value measured for PD was 9 kPa recorded at a weight fraction of 0.15 and Re=13500, which was 28% larger than that of measured for the base fluid. It was also found that a continual fouling layer of nanoparticles (NPs) was formed on the boiling surface, which induced a thermal resistance against the boiling heat transfer. The fouling formation reduced the HTC of the NF such that the maximum reduction in the HTC was 21.6% after 1000 minutes of the operation of the heater.

Investigation of heat transfer characteristics of high-altitude intercooler for piston aero-engine based on multi-scale coupling method

Altitude affects the heat exchange capacity of a heat exchanger, which in turn affects the performance of an aircraft with piston engines. In order to study this mechanism of influence and the associated principles, a three-dimensional computational fluid dynamics (CFD) simulation of an intercooler is conducted based on multi-scale coupling. The simulation results have a high degree of coincidence with the test data on ground, with maximum error of no more than 10%. The simulation height is then extended to an altitude of 20 km. It is found that before the aircraft enters the stratosphere, the overall efficiency of the intercooler decreases by 1.98% on average for every 1km increase in the altitude and decreases by 3.26% when the height is above 11 km. The heat transfer capacity of the intercooler is gradually enhanced owing to the gradually increasing temperature difference while atmospheric density is relatively large. After entering the stratosphere, the external temperature stops changing; the density decreases to 1/14 of the value at the ground, and the low density becomes the dominant factor, resulting in reduced heat transfer coefficient. Finally, an empirical formula for the overall heat exchange efficiency of the intercooler as a function of the altitude is proposed.

Augmentation of natural convection heat sink via using displacement design

Miniaturization of electronic devices demands an effective thermal management system for efficient cooling. Still, many electronic industries use natural convection cooling techniques like plate-fin heat sinks as far as no moving part (noise) and reliability is concerned. In this study, the widely-used plate fin heat sink is augmented by a novel fin displacement amid fin array. Yet the effect of fin displacement between alternate fins is experimentally and numerically investigated under natural convection condition. In the meantime, the effects of fin spacing, length, height, and heat flux on the heat sink performance subject to fin displacement is also studied. For the smaller fin pacing, introducing the fin displacement will delay the merging of boundary layers, and reduce the length of fully developed region accordingly. Smaller fin spacing can also entrain the outside air into the fin array at the upper portion of heat sink for offering additional enhancement. The fin displacement is especially effective for a smaller fin spacing. This is because fully developed flow prevails at a smaller fin spacing. On the other hand, the fin displacement may impair the performance when a very wide fin spacing is used. Similarly, the increase in the fin length also enhances the heat transfer performance under the fin displacement conditions. It is also found that the effect of fin height on the thermal performance is small and the effect of heat flux is negligible. The maximum reduction in the thermal resistance is 56% by adopting the displacement design. A drop in fin temperature at the upper portion with the displacement design may lead to a reduction in heat transfer coefficient. For the comparison containing the fixed volume, the heat sink with fin displacement offers a 30% heat transfer enhancement ratio, 28.7% reduction in total mass, and 27.4 % reduction in surface area when compared to the conventional heat sinks.

An experimental investigation of the effect of multiple inlet restrictors on the heat transfer and pressure drop in a flow boiling microchannel heat sink

Flow boiling instabilities inherent in microchannels / microgaps remain to be a serious issue in two-phase high-heat-flux cooling applications, as they cause significant oscillations in flow rate, temperature, pressure, reduction in heat transfer coefficient (HTC), and eventually early occurrence of critical heat flux (CHF). This study experimentally investigated the effects of various configurations of inlet restrictors (IRs) on the thermal hydraulic performance of flow boiling in a microchannel heat sink, which has a single rectangular microchannel with an aspect ratio of 13.12 and a hydraulic diameter of the 708 µm. The experiments were carried out for the microchannel with three designs of inlet restrictors: one-slot opening (1IR), three-slot openings (3IR), and five-slot openings (5IR), for mass fluxes of 32.68, 81.29 and 144 kg/m2 s, respectively. The effects of the various IRs on the CHF, average HTC and pressure drops of the microchannel heat sink were analyzed. The results showed that all test cases with IRs improved the CHF performance of the flow boiling microchannel heat sink, where the 5IR case works best at low mass flux and the 1IR case works best at high mass flux. The results also illustrated that the IRs reduce the HTC at low mass flux, but improve the HTC at high mass flux and heat flux. IRs always exhibit higher pressure drop penalties. This study also revealed the optimum design for microchannel with IRs, which depends on the operational parameters (e.g., mass flux and heat flux) of the microchannel heat sink.

Metrological performances of fouling sensors based on steady thermal excitation applied to bioprocess

Food and industrial bioprocesses are impacted by (bio)fouling which generates failures from reduction of process efficiency (ex: reduction of heat transfer coefficient) up to health risk issue (e.g. biofilm formation). In present work, 3 fouling sensors based on a thermal excitation (steady thermal regime) were developed and described. These sensors were designed with different technologies (macro structure and Micro-Electro-Mechanical-Systems MEMS), geometries (intrusive cylindrical, flush plan) and packaging (presence or absence of cover panel) and compared. Laboratory setups were designed to characterize sensor responses under controlled operating conditions in batch and continuous process including clean condition and using layers of adhesive tape to simulate fouled conditions. Thermal responses from excitation under steady thermal regime at different heat flux were linearized then discussed as function of technology, geometry and packaging impacts. Packaging heat resistance, response times, efficient heat flux, and quantification of fouling were investigated. Finally, metrological limits were identified.