Heat-up Period Research Articles

Investigating the potential of a higher reactivity fuel to achieve faster heat-up of aftertreatment systems

Shortening the aftertreatment system heat-up period will be crucial to meet upcoming emissions regulations, which mandate significant reductions in tail-pipe emissions for on-road heavy-duty vehicles. This work studies the impact of increasing fuel reactivity on the combustion of post injections during the expansion stroke. The work demonstrates an operating strategy using a blend of di-butyl ether (a potential low-carbon biofuel component) and #2 diesel that achieves higher exhaust enthalpy, while maintaining similar engine-out emissions (NOx and combustion efficiency) in comparison to the baseline #2 diesel fuel (cetane number 41). On average, a 12% increase in exhaust enthalpy was observed for the blend (cetane number 60) relative to the baseline fuel. The ability to operate the higher reactivity fuel at a more retarded post-injection timing, where combustion with #2 diesel was found to be more unstable, and less complete was key to achieving higher exhaust enthalpy. However, a small fuel penalty (∼ 3%–5%) was also associated with the higher reactivity fuel with this operating strategy. The formation of fuel lean regions, due to the long ignition delays at late injection conditions, was predicted to be the leading contributor to the increased emissions of products of partial combustion. Chemical kinetic simulations using surrogates for the fuels used in the experiments, demonstrated the ability of a higher cetane fuel to outperform the baseline fuel under fuel lean conditions with equivalence ratios less than 0.7.

Heating and evaporation of a mono-component spheroidal droplet with non-uniform surface temperature

A new mathematical model for spheroidal droplet heating and evaporation is proposed. This model takes into account the effect of liquid finite thermal conductivity and is based on the previously obtained analytical solution for the vapour mass fraction at the droplet surface and a new correlation for the convective heat transfer coefficient incorporated into the numerical code. The heat transfer equation in the liquid phase is solved numerically using the finite-element heat transfer module of COMSOL Multiphysics. It is shown that the lifetime of spheroidal (prolate and oblate) droplets is shorter than that of spherical droplets of the same volume. The difference in the lifetimes of spheroidal and spherical droplets, predicted by the new model, is shown to increase with increasing aspect ratios for prolate droplets and decreasing aspect ratios for oblate droplets. As in the case of stationary spherical droplets, the d2-law is shown to be valid for spheroidal droplets after the completion of the heat-up period. The predictions of this model agree with experimental observations. The duration of the heat-up period is shown to decrease with increasing aspect ratios for prolate droplets and decreasing aspect ratios for oblate droplets. The maximal surface temperatures are predicted near the regions where the surface curvature is maximal. The aspect ratios are shown to be weak functions of time, in agreement with experimental observations.

Sodium salt surface interactions affect single particle combustion behavior of biomass, coal, and their chars

Sodium salt additions provide an effective route for tuning the combustion behavior of solid fuels. The salt-fuel surface interactions play a key role in combustion, as demonstrated by low heating rate experiments. High heating rate single particle studies have not investigated the effect of surface interactions and microstructural changes for sodium salt-treated fuels. In this work, the effect of three different sodium salt treatments were investigated on the single particle combustion characteristics of two distinct solid fuels and their corresponding chars. This approach probed the influence of microstructural changes and the formation of oxidation-resistive functional groups during high heating rate combustion. The separate characterization of biomass, coal, and their respective chars aimed at identifying the effect of sodium during different stages of combustion. The investigation encompassed Tunçbilek lignite (TL), olive residue (OR), and their fast pyrolysis chars (TLc and ORc) obtained from a wire mesh reactor at 600 °C. Both pristine and char particles were treated with Na2CO3, NaOH, and NaAlO2 to investigate the catalytic/inhibiting effects at different stages of combustion, namely, ignition, volatile combustion, and char combustion. All samples were burned as single particles at a high heating rate, WMR (1600 °C s−1). Treated OR samples provided earlier ignition reaching ∼ 0.4 s for OR-OH and shorter char combustion times around ∼ 2.8 s mg−1 for OR-CO3. On the contrary, treated TL samples depicted slightly higher ignition delay (3.0 s for TL-OH) and prolonged char combustion (11.2 s mg−1 for TL-OH) compared to TL (2.3 s, 8.2 s mg−1). Treated TLc samples also exhibited higher ignition delay due to the hygroscopic behavior of the sodium salts, which is more pronounced in TLc-OH (5.5 s). All treated chars exhibited shorter char combustion times except TLc-OH. Overall, sodium salt additions improved characteristic combustion times of biomass and chars while inhibiting those of lignites. The inhibitive effect of sodium salts on treated OR and ORc samples was more pronounced for TGA experiments due to the alkali interaction with the lignocellulosic component and a prolonged heating-up period. The findings demonstrate the importance of high heating rate characterization to accurately understand the sodium salt additions on combustion performance.

Evaporation characteristics of ethanol droplet in different gases under force convection condition

Ethanol is recognized as one of the finest alternative bio-fuels due to its natural characteristics. Fundamental studies on ethanol droplet evaporation process are mainly either in stagnant environment or using some semi-empirical co-relations. Here, a fully numerical model based on the first principle is solved to investigate the effects of various atmospheric gases (Ar, N2, O2 and CO2) on droplet evaporation phenomenon under force convective situation. Two-dimensional governing equations of species, momentum and energy transfer of spherical coordinate system are solved, and the simulation is validated quantitatively with the literature. Uniform convective strength (Re = 100) is maintained for all cases examined at T∞ = 500 K and P∞ = 0.1 MPa. From the simulation results, it is observed that the viscosity ratio (liquid to gas) has effect on droplet life time. The ethanol droplet life time increases in less viscous atmospheric gases. The ethanol droplet life time is shorter in Ar gas, but heat-up period, wet-bulb temperature, and the surface blowing effect are higher in Ar compared to other atmospheric gases. The heat-up periods of the ethanol droplet in Ar, CO2, N2 and O2 atmosphere occupy around 30%, 20%, 17% and 16.5% of the total life time of the droplet, respectively. It is also noticed that, the heat-up period increases with increase in the thermal conductivity ratio (liquid to gas) and vice versa. Furthermore, the flow pattern of both gas- and liquid-phase in terms of streamline and the internal temperature distributions of the droplet are visualized at various time instants.

Investigation of supercritical transition and evaporation process of a hydrocarbon droplet under diesel engine-relevant conditions

Fuel spray supercritical transition in the diesel engine has been concerned widely, especially supercharged ones, however, whether the transition can significantly affect the fuel–air mixing under diesel engine-relevant conditions is an issue that still needs evaluating carefully. Thus, the transition and evaporation of individual n-heptane droplets in a nitrogen environment under various ambient conditions were investigated through a high-pressure evaporation model. The influence of ambient pressures on droplet lifetime (τlife) and evaporation rate constant (K) varies with ambient temperature. When Tr < 0.98, with increasing pressure, τlife first increases and then decreases, and K decreases. When 0.98 < Tr < 1.28, as pressure increases, τlife would increase even if K increases, because the increment of initial heat-up periods exceeds the decrease in quasi-steady evaporation periods. When Tr > 1.28, with increasing pressure, τlife decreases, and K increases. As the droplet heats up, the droplet interface is where the binary mixture would reach the critical mixing state if ambient conditions are high enough. Increasing ambient density decreases the minimum ambient temperature (Tm) required for the transition to occur. Above the Tm, transitions occur earlier with the increasing energy density of ambient gas. Increasing ambient density decreases the minimum ambient temperature required for diffusive fuel-gas mixing modes to dominate the mixing process. For droplets large enough to ignore gas–liquid interfacial thickness, the Tm required for the attainment of critical mixing states is almost independent of droplet diameter.

Ignition of thermally-thick blunt body PMMA samples using a heated wire

Ignition limits and ignition delay times of thermally-thick polymethyl methacrylate (PMMA) spheres (4-cm diameter) and cylinders (5.08-cm diameter) are investigated by placing an electrically-heated wire coil beneath the sample in a slow forced flow. The wire serves as both a heater for the solid sample and a gas-phase hot spot to ignite the oxidizer-fuel vapor mixture. In the experiments, the wire's electrical current and power-on time, its distance from the sample surface, the forced flow velocity, oxygen percentage, and pressure are systematically varied. Ignition limits as a function of the oxygen percentage, pressure, and igniter current are determined. Two ignition delay times are determined: igniter-assisted ignition and self-sustained ignition. These critical times are also functions of the degree of heat-up of the sample interior. Solid temperature profiles are measured using embedded thermocouples to determine the sub-surface temperature gradients at the moment of ignition. The heat transfer modes from the igniter to the solid sample are numerically investigated during the pre-pyrolysis heat-up period. At the forward stagnation-point region where ignition occurs, radiation is greater than convection and accounts for approximately 60% of the total heat input. This is consistent with the experimental findings on ignition delay times and heat flux measurements.

Experimental study on improved performance of an automotive CO2 air conditioning system with an evaporative gas cooler

A well-known challenge in the application of carbon dioxide (CO2) refrigerant for vehicles is the great energy efficiency attenuation in hot climates. In attempting to solve this problem, a novel design integrating evaporative cooling with automotive CO2 air conditioning system, where water was sprayed to the gas cooler, was introduced and experimentally investigated in present work. Results illustrate that with this design the CO2 temperature leaving gas cooler could be reduced to a lower level or even below the outdoor air temperature, thus inducing a significant performance improvement. A coefficient of performance (COP) enhancement percentage of 42.4% was achieved at 35°C outdoor condition. This percentage was increased up to 52.9% when outdoor air temperature increased to 45°C. With the decreased outdoor air velocity and the decreased outdoor air humidity, the merit of this design was more significant. These indicate that the evaporative gas cooler is a promising technology to alleviate the problem of performance deterioration in hot and dry weather conditions. A reduced spray water temperature could not cause a significant change in the COP enhancement. This may be attributed to that the sensible heat during droplet heat-up period was negligible compared to the latent heat of droplet evaporation. When the surface position of gas cooler chosen to be sprayed was moved from CO2 inlet to CO2 outlet, the COP enhancement increased, which suggests that water should be sprayed to the area where wall temperature is low.

Numerical investigation of multi-component droplet evaporation and autoignition for aero-engine applications

The droplet evaporation and autoignition behaviour of a three-component kerosene surrogate is numerically investigated for a wide range of ambient pressures and temperatures representing realistic aero-engine conditions. Vitiated air with different levels of dilution is considered to represent mixing of air with combustion products. Particular attention is given to the analysis of multi-component fuel effects at the various operating conditions. The investigation also considers the impact of fuel preheating and multiple initial droplet diameters, and extends to the quantification of NOx emissions at the droplet scale level. Considering pure evaporation, results show that preferential evaporation and variation of the droplet composition are mainly affected by the gas temperature. An increase of the pressure generally increases the duration of the droplet heat-up period and reduces the effects of preferential evaporation, especially when high temperatures are considered. Autoignition in vitiated air is strongly influenced by both the level of dilution and ambient pressure, with the latter playing an important role in determining the value of the initial droplet diameter below which no autoignition occurs. Lower pressures generally make the kerosene droplet more resistant to autoignition for the same level of dilution or gas temperature. Droplet preheating mainly affects the heat-up period and can be used as a design parameter to control the autoignition delay time. NOx levels are substantially related to the gas-phase temperature and the existence of a flame at the droplet scale. Potential implications for the design of future low-emission combustor technologies are discussed with an emphasis on the fuel preparation.

Computational Modeling Approaches of Hydrothermal Carbonization: A Critical Review

Hydrothermal carbonization (HTC) continues to gain recognition over other valorization techniques for organic and biomass residue in recent research. The hydrochar product of HTC can be effectively produced from various sustainable resources and has been shown to have impressive potential for a wide range of applications. As industries work to adapt the implementation of HTC over large processes, the need for reliable models that can be referred to for predictions and optimization studies are becoming imperative. Although much of the available research relating to HTC has worked on the modeling area, a large gap remains in developing advanced computational models that can better describe the complex mechanisms, heat transfer, and fluid dynamics that take place in the reactor of the process. This review aims to highlight the importance of expanding the research relating to computational modeling for HTC conversion of biomass. It identifies six research areas that are recommended to be further examined for contributing to necessary advancements that need to be made for large-scale and continuous HTC operations. The six areas that are identified for further investigation are variable feedstock compositions, heat of exothermic reactions, type of reactor and scale-up, consideration of pre-pressurization, consideration of the heat-up period, and porosity of feedstock. Addressing these areas in future HTC modeling efforts will greatly help with commercialization of this promising technology.

D2 Law and Penetration Length of Jatropha and Camelina Bio-Synthetic Paraffinic Kerosene Spray Characteristics at Take-Off, Top of Climb and Cruise

A comparison of d2 law and penetration length of biofuels with Jet–A through the incorporation of fuel properties and actual combustor inlet data at various flight trajectories is presented. This study aims to identify fuel properties and flight operating conditions that most influence droplet characteristics accurately. The study comprises two phases involving a simulation using GSP to predict combustor inlet data for the respective flight operating conditions and a simulation using ANSYS Fluent V18.1 to obtain combustion characteristics of biofuels and Jet–A. The biofuels chosen in this study are Jatropha Bio-synthetic Paraffinic Kerosene (JSPK) and Camelina Bio-synthetic Paraffinic Kerosene (CSPK), evaluated as pure (100%) and blend (50%) with Jet–A. Thrust specific fuel consumption (TSFC) of biofuels is improved due to lower fuel consumed by the engine. The d2 law curve shows a heat-up period that takes place at the early stage of the combustion process. The penetration length of the fuels is shorter at take-off. Combusting biofuels reduce combustion temperature and the penetration length of the droplet.

A comparative study on direct liquefaction of two coals and hydrogen efficiency to the main products

To improve the performance of megaton direct coal liquefaction (DCL) technology that has been practiced in China since 2008 and to develop a better technology that can be operated at milder conditions with high efficiency, this work studies coal conversion (XC), products yield (Yi) and hydrogen consumption (RH) of SW coal (H/C = 0.75) used in the megaton plant and NMH coal of a higher H/C ratio (0.97) in tetralin and a temperature range of 380–455 °C. The experiments were carried out in fast heating high-throughput micro reactors and the dependencies of XC and Yi on temperature, time, and RH are studied. The results show that the XC-RH and Yi-RH relations are coal dependent but not temperature dependent in the temperature range of 380–440 °C. The kinetics modeling indicates that NMH contains more mobile fractions than SW and appreciable amounts of coals react in the 1 min heating-up period. The rate constants of coal to each product follows the order of asphalt (AS) > oil (O) > organic gas (OG), while the activation energy and H consumption to each product follows a reverse order.

Low-temperature oxidation and self-heating accelerated spontaneous combustion properties of a Yima formation bituminous coal with various moisture contents

By thermogravimetry (TG) experiments, evolution behaviors and properties of thermal oxidation processes at low temperature (< 230.0 °C) of a bituminous coal with five moisture contents were investigated. A novel Acceleration Temperature Point Method (ATPM), defined as the transition point from low-temperature oxidation to rapid self-sustained heating-up period, was proposed to determine the role of moisture content in accelerating such process. Results show that segments of water evaporation and gas desorption dominate in stage 1 (65.0–150.0 °C), while segments of generation of coal-oxygen complex and structure oxidation dominate in stage 2 (150.0–275.0 °C). In the moisture range of 9.16–12.06%, the promoted effect of physical desorption capacity becomes weak with increasing moisture content. This is owing to five aspects: the equilibrium of vapor pressure and wetting capability of physically bound water, the change of mass and heat transfer between gas phase (water vapor)-condensed phase (bulk coal particle), the diffusing effect of gaseous water, the heat from dissolving and swelling of gaseous water into bulk coal pores, and the heat of pyrolysis derived from coal particles. Taking Acceleration Temperature Point (ATP) as a barometer to evaluate the spontaneous combustion susceptibility, the critical moisture content of Gengcun bituminous coal is 9.16%.

High Temperature Conductive Stability of Indium Tin Oxide Films

Indium tin oxide (ITO) has been studied in applications at normal high temperature below 600 °C, due to excellently electrical characteristics. Attempts to further match the needs of electronics in extremely harsh environments, the changes in the conductive properties of ITO films and its mechanism were investigated at special high-temperature above 1000 °C. ITO films were prepared by pulsed laser deposition (PLA) on lanthanum gallium silicate (LGS) substrates. Furthermore, the as-deposited samples were annealed at different temperature-time treatments, and investigated the effects of annealing on the electrical, structure, surface morphology and chemical properties of ITO films by X-ray diffraction analysis, scanning electron microscopy (SEM), resistance measurement, and X-ray photoelectron spectroscopy (XPS). The experimental results present that the decreased resistance of ITO films was mainly attributed to the increase of the crystalline size and the increased amount of Sn 4+ ions during the heating-up period (0 °C to 1000 °C). Generally, The ITO film showed stable electrical properties when it was heated at the temperature of 1000 °C for at least 2.5hours. The expected ITO films that remain steady above 1000 °C have potential applications as electrodes working in special high-temperature environments.

The drying process of Sarcocornia perennis: impact on nutritional and physico-chemical properties.

The Sarcocornia genus is an extreme salt-tolerant plant that can be cultivated in saline habitats almost worldwide. To preserve Sarcocornia perennis, convective drying experiments were conducted and their effects on the physico-chemical properties and phenolic content of the plant were studied using conventional and vibrational spectroscopy techniques. The drying process of Sarcocornia perennis at temperatures of 40°C, 50°C, 60°C and 70°C revealed three periods of convective drying process with drying times ranging between 4.5 and 24.9h, respectively to higher and lower temperatures. The heating-up period can be neglected as compared with the drying process, and the duration of constant rate period, as a percentage of the total drying time, ranged between 34 and 20% respectively at 40°C and 70°C. The Modified Page model was proposed to describe the drying process at the different temperatures. From a nutritional point of view, this halophyte plant may be considered as a good source of fibres, phenolic compounds and natural minerals, such as sodium, potassium, calcium and magnesium. The convective drying, in the temperature range currently used, was found to preserve the colour, nutritional characteristics and phytochemical value of Sarcocornia perennis. These results were confirmed by FTIR-ATR and highlight the potential use of the dried plant in novel food products.

Evaporation of a sessile binary droplet on a heated spherical particle

This study investigated the evaporation behaviour of a binary droplet system (diameter: ∼2.6–2.9 mm) comprising water-glycerol mixture (0–35 wt.%) in contact with a spherical particle heated in the range from 323 to 358 K below the saturation temperature of the mixture. Specifically, the effect of liquid composition and particle temperature on both the evaporation rate and temporal variation in droplet temperature were studied. Different approaches to quantify droplet evaporation rate in terms of reduction in volume, equivalent diameter and spherical cap height were analysed. A fairly linear evaporation rate was obtained in all the cases studied. Further, temporal measurements of temperature were obtained at several locations within the droplet under different particle surface temperatures which revealed a short unsteady heating-up period followed by a longer steady state stage. A scaling analysis was performed to predict the unsteady heating time by the thermal diffusion time scale including and excluding the effect of internal convection. Also studied in this work was the variation in surface wettability which was characterised by the wetted area diameter and contact angle. Transient contact angle predicted based on the empirical evaporation model incorporating the Marangoni effect produced good agreement with the experimental data. A linear correlation between the normalised wetted area diameter and the apparent contact angle was obtained. Finally, a brief scaling analysis was presented to quantify various internal motions which showed relative dominance of Marangoni flows in the enhancement of evaporation rate.

Fume emissions from a low-cost 3-D printer with various filaments

ABSTRACT3-D printing is an additive manufacturing process involving the injection of melted thermoplastic polymers, which are then laid down in layers to achieve a pre-designed shape. The heated deposition process raises concerns of potential aerosol and volatile organic compounds (VOC) emission and exposure. The decreasing cost of desktop 3-D printers has made the use of 3-D printers more acceptable in non-industrial workplaces lacking sufficient ventilation. Meanwhile, little is known about the characteristics of 3-D printing fume emission. The objective of this study was to characterize aerosols and VOC emissions generated from various filaments used with a low-cost 3-D printer in an environmental testing chamber. A pre-designed object was printed in 1.25 hours using eight types of filaments. A scanning mobility particle sizer and an aerodynamic particle sizer were employed to measure the particle size distribution in sub-half-micron fraction (<0.5 µm) and super-half-micron fraction (0.5–20 µm), respectively. VOC concentration was monitored real-time by a photoionization detector and sampled with a tri-sorbent thermal desorption tube, and analyzed by thermal desorption gas chromatography mass spectrometry (TD-GC/MS). Results showed high levels of fume particles emission rate (1.0 × 107 to 1.2 × 1010 #/min) in the sub-half-micron range with mode sizes of 41–83 nm. Particle concentrations peaked during the heat-up and solid layer printing periods. Total VOC concentration in the chamber followed a first-order buildup, with predominant VOC species in the chamber were breakdown and reaction products of the filaments, such as styrene from ABS filaments. These findings and exposure scenario estimation suggest that although the VOC concentrations were much lower than occupational exposure limits, particles with size less than micron might be a concern for users of low-cost 3-D printers due to high respirablity, especially if used in settings without proper guidance and engineering control.

Preparation of calcium peroxide in a UHF field at atmospheric pressure

The kinetics of free moisture removal from a CaO2 paste under the conditions of convective drying and UHF field at atmospheric pressure were compared. Drying under the action of UHF radiation is characterized by the absence of the heat-up period. The use of the microwave radiation at atmospheric pressure decreases the drying time in production of anhydrous calcium peroxide by a factor of approximately 10 compared to convective drying.

Thermal Emission of Alkali Metal Ions from Al30-Pillared Montmorillonite Studied by Mass Spectrometric Method.

The thermal emission of alkali metal ions from Al30-pillared montmorillonite in comparison with its natural form was studied by mass spectrometry in the temperature range 770–930 K. The measurements were carried out on a magnetic mass spectrometer MI-1201. For natural montmorillonite, the densities of the emission currents (j) decrease in the mass spectrum in the following sequence (T = 805 K, A/cm2): K+ (4.55 · 10−14), Cs+ (9.72 · 10−15), Rb+ (1.13 · 10−15), Na+ (1.75 · 10−16), Li+ (3.37 · 10−17). For Al30-pillared montmorillonite, thermionic emission undergoes temperature-time changes. In the low-temperature section of the investigated range (770–805 K), the value of j increases substantially for all ions in comparison with natural montmorillonite (T = 805 K, A/cm2): Cs+ (6.47 · 10−13), K+ (9.44 · 10−14), Na+ (3.34 · 10−15), Rb+ (1.77 · 10−15), and Li+ (4.59 · 10−16). A reversible anomaly is observed in the temperature range 805–832 K: with increasing temperature, the value of j of alkaline ions falls abruptly. This effect increases with increasing ionic radius of M+. After a long heating-up period, this anomaly disappears and the lnj − 1/T dependence acquires a classical linear form. The results are interpreted from the point of view of the dependence of the efficiency of thermionic emission on the phase transformations of pillars.

Microgravity experiments of fuel droplet evaporation in sub- and supercritical environments

Droplet evaporation in sub- and supercritical environments has been studied experimentally under microgravity conditions. A single suspended droplet of n-hexadecane was employed for the experiments. The initial droplet diameter was 0.4mm. A pair of alumina/silica fibers of 7µm in diameter was applied to suspend a droplet. The ambient pressure was varied in the range of 1.0–3.0MPa, and the ambient temperature was set at 773K. Sequential backlit images of an evaporating droplet were recorded using a high-speed digital video camera. Temporal variations in the droplet diameter were measured using a self-made computer-aided image analyzer. Microgravity conditions were produced by a 50-m drop tower. Temporal variations in the droplet diameter were successfully obtained for droplet evaporations in the supercritical environments. The normalized droplet lifetime increased with the ambient pressure. The evaporation rate constant increased with the ambient pressure, reached the maximum value at an ambient pressure slightly above the critical pressure of the fuel, and then decreased. The initial heat-up period linearly increased with the ambient pressure, reached the maximum value at an ambient pressure of 2.0MPa, and then decreased. The ratio of the initial heat-up period to droplet lifetime increased with the ambient pressure, reached the maximum value of about 0.6 at an ambient pressure of 2.0MPa, and then decreased. The droplet evaporation lifetime increased with the ambient pressure at subcritical ambient pressures even though the evaporation rate constant increased because the increase in the initial heat-up period overtook the decrease in the quasi-steady evaporation period. It was found that, in the case of fuels with a high critical temperature, the initial heat-up period determines the ambient pressure dependence of the droplet evaporation lifetime in the environments around the critical point of the fuel.

Experimental studies on natural convection of water in a closed-loop vertical annulus

Experimental studies on heat transfer and fluid flow of water in a vertical annulus, circulating through a cold leg forming a closed loop thermo-siphon, have been carried out in this article. The annulus has a radius ratio (outer radius to inner radius) of 1.184 and aspect ratio (length to annular gap) equal to 352. The experiments were conducted for constant heat fluxes of 1, 2.5, 5, 7.5, 10, 12.5, and 15 kW/m2. Transient behavior during the heat-up period of the system until the steady-state condition is attained and discussed. Variation in the heat transfer coefficient and Nusselt number along the annulus height represent the developing boundary layer at the entrance and fully developed flow in the remaining length. A large drop in the differential pressure is experienced when the liquid is circulated through the flow meters, which restrict the flow due to their very small passages. Flow restriction causes mass accumulation and rise of pressure at the exit of the annulus. It also causes a decrease in liquid head in the cooling leg. An increase in the heat flux leads to an increase in the heat transfer coefficient and Nusselt number. As a result of the data analysis correlations for the average Nusselt number, Reynolds number and circulation rate have been developed in terms of the heat flux.