Single-phase Heat Transfer Coefficient Research Articles

Confined jet array impingement boiling

Confined jet array impingement boiling heat transfer is investigated for water at atmospheric pressure and inlet subcooling of 8°C for flow rates ranging between 0.33 and 0.67L/min. Three jet configurations consisting of 2×2, 3×3 and 5×5 arrays of 1.0mm diameter jets were tested for a jet-to-target spacing of 2mm. A 15mm×15mm plane copper surface was used as the heat transfer surface which formed a confined channel with the upper jet orifice plate. For a Reynolds number (Re) range of 900≤Re≤11,800, tests were performed by fixing the flow rate and progressively increasing the heat flux until the Critical Heat Flux (CHF) was reached. The results show that the single phase heat transfer coefficient increases with increasing Re and is reasonably predicted by a recent jet array heat transfer correlation. However, once fully developed boiling conditions are achieved, the heat transfer is generally insensitive to the flow rate and is predicted by the well-established Cooper pool boiling correlation. Consistent with earlier confined boiling studies, this suggests that the dominant heat transfer mechanisms are associated with the local bubble activity opposed to convective influences. Contrastingly, both the Onset of Nucleate Boiling (ONB) and CHF were found to be strongly dependent on the jet velocity. For the latter, it is related to the ability of the flow to clear the confined channel of vapour. CHF predictions are compared with the measurements showing good agreement.

Analysis and comparison of internal and external temperature measurements of a tubular oscillating heat pipe

The current study examines the relationship between internal/fluidic and external/wall temperature measurements along the adiabatic section of an operating tubular oscillating heat pipe (T-OHP) for varying heat inputs. Temperature measurements were achieved using type-T thermocouples located either inside or along the OHP wall in the region between the evaporator and condenser. Measurements were utilized to elucidate the effects of wall thermal capacitance, external wall temperature gradient, and internal fluid advection. The internal, single-phase heat transfer coefficient was estimated, and the effective thermal conductivity of the OHP was determined. A 4-turn copper T-OHP (3.25mm ID) was charged with water (75% by volume) and tested in the bottom-heating condition. Heat input was varied in increments of 25W from 60W to 300W. Results indicate that the external thermocouples were unable to capture frequency components larger than ∼1Hz. Internal measurements indicate that average, evaporator-side fluid oscillation frequencies varied from ∼1.5Hz at 60W to ∼2.5Hz at 300W, whereas condenser-side frequencies remained fairly constant at ∼0.5Hz. The frequency transfer function corresponding to the thermal resistance network between the internal/external thermocouples was found to be constant across all tested power inputs. The low-frequency, large-amplitude changes in internal temperature associated with bulk fluid motion were not immediately measured at the external OHP tube surface. The effective thermal conductivity calculated using only external temperature measurements was found to be 4–12% lower than that calculated using internal measurements. The maximum, calculated effective thermal conductivity using internal or external temperature measurements was 15,300W/m·K and 14,000W/m·K, respectively. This difference arises from there being a smaller, length-wise temperature gradient along the fluid columns than along the tube wall due to the strong advection component of OHP heat transfer. Tube wall conduction was found to account for 2–10% of the overall heat transfer, with its significance decreasing as fluid advection increased at higher heat inputs. The heat transfer coefficient for single-phase fluid oscillation inside the OHP was estimated to be ∼1000W/m2K for power inputs larger than 100W; corresponding to Nusselt numbers between 4 and 6.

Single-phase ambient and cryogenic temperature heat transfer coefficients in microchannels

Micro-scaling cryogenic refrigerators, in particular the Joule-Thomson (JT) variety require very good information about heat transfer characteristics of the refrigerants flowing in the microchannels for optimal design and performance. The extremely low Reynolds flow is present in a micro JT cryocooler, the heat transfer characteristics at these conditions require investigation. There are numerous studies regarding heat transfer coefficient measurements of liquid flow in microchannels at/near ambient temperature and high Reynolds flow (Re>2000), that agree well with the conventional correlations. However, results from previous studies of gaseous flow in microchannels at low Reynolds flow (Re<1000) disagree with conventional theory. Moreover, the studies performed at cryogenic temperatures are quite limited in number. In this paper, the single-phase heat transfer coefficients and friction factors for nitrogen are measured at ambient and cryogenic temperatures. The hydraulic diameters for this study are 60, 110 and 180 μm for circular microchannels. The Reynolds numbers varied from a very low value of 10 to 3000. The measured friction factors are comparable to those in macro-scale tubes. The experimental results of the heat transfer indicate that Nusselt numbers derived from measurements are significantly affected by axial conduction at low Reynolds flow (Re<500). The Nusselt numbers at high Reynolds flow (Re>1000) follow conventional theory. The detailed experiment, procedure, and measured results are presented in this paper and discussed regarding deviation from ideal theory at low Reynolds flow.

Boiling of R134a in a Plate-Fin Heat Exchanger Having Offset Fins

This paper presents experimental results on boiling heat transfer of R134a in a compact plate fin heat exchanger. The exchanger is made of aluminum and has high density offset fins (30 fins/in.). Such heat exchangers are widely used in air separation industry and aerospace applications because of their high compactness and low weight. The test heat exchanger is attached to a vapor cycle refrigeration basic module to study the effects of boiling phenomena and its influence on performance as there is limited information available for this type of fins. This in turn allows for discussion on boiling mechanism of R134a inside the fins using the water circuit on the other side of the test heat exchanger. The water side single phase heat transfer coefficient (Colburn j factor) is calculated using the cfd tool fluent and validated with available open literature. The results are presented for heat fluxes up to 5500 W/m2 and mass fluxes up to 20 kg/(m2s) with water side flow rate varying from 0.033 to 0.17 kg/s for water temperatures of 10, 15, 20, 25, and 30 °C.

Numerical Analyses of Single-Phase Pressure Drop and Forced Convective Heat Transfer Coefficient of Water-Ethanol Mixture: An Application in Cooling of HEV Battery Module

The present numerical analyses are related to the cooling of a hybrid electric vehicle (HEV) battery module by water–ethanol mixture. The fluid is passed through a cold plate consisting of two rectangular channels of 0.01 m depth, 0.015 m width, and 0.15 m length. The battery module is represented by a heater placed below the cold plate. The single-phase pressure drop and single-phase heat transfer coefficient for water, water–ethanol mixture of mass fraction of 25%, 50%, and 75%, and ethanol are determined numerically for different heat fluxes of 10, 15, 20, and 25 kW/m2 and different Reynolds numbers 500, 1000, 1500, 2000, and 2500. To solve the Navier–Stokes equation, the pressure correction method was used and to solve the energy equation, the Lax–Wendroff explicit method is used. Numerical results obtained for water are compared with the literature correlations. The friction factor for water deviated by an average of 8.02% from the Lewis and Robertson equation. The Nusselt number for water deviated by 7.35% from the Churchill and Ozoe equation at lower Reynolds number 500 and at higher Reynolds number 2500, Nusselt number deviated by 13.68% from the Stephan equation. The results showed that the heat transfer coefficient increased with an increase in Reynolds number and heat flux. The effect of the increase in Reynolds number is more significant than the increase in heat flux. At higher ethanol mass fraction and higher Reynolds number the heat transfer coefficient increased with heat flux when compared to water. There is no significant decrease in heat transfer coefficient with an increase in ethanol mass fraction. The pressure drop increased and the heat transfer coefficient decreased with an increase in ethanol mass fraction.

Boiling Heat Transfer From an Array of Round Jets With Hybrid Surface Enhancements

The effect of a variety of surface enhancements on the heat transfer achieved with an array of impinging jets is experimentally investigated using the dielectric fluid HFE-7100 at different volumetric flow rates. The performance of a 5 × 5 array of jets, each 0.75 mm in diameter, is compared to that of a single 3.75 mm diameter jet with the same total open orifice area, in single-and two-phase operation. Four different target copper surfaces are evaluated: a baseline smooth flat surface, a flat surface coated with a microporous layer, a surface with macroscale area enhancement (extended square pin–fins), and a hybrid surface on which the pin–fins are coated with the microporous layer; area-averaged heat transfer and pressure drop measurements are reported. The array of jets enhances the single-phase heat transfer coefficients by 1.13–1.29 times and extends the critical heat flux (CHF) on all surfaces compared to the single jet at the same volumetric flow rates. Additionally, the array greatly enhances the heat flux dissipation capability of the hybrid coated pin–fin surface, extending CHF by 1.89–2.33 times compared to the single jet on this surface, with a minimal increase in pressure drop. The jet array coupled with the hybrid enhancement dissipates a maximum heat flux of 205.8 W/cm2 (heat input of 1.33 kW) at a flow rate of 1800 ml/min (corresponding to a jet diameter-based Reynolds number of 7800) with a pressure drop incurred of only 10.9 kPa. Compared to the single jet impinging on the smooth flat surface, the array of jets on the coated pin–fin enhanced surface increased CHF by a factor of over four at all flow rates.

Two-Phase Experimental Studies on Liquid–Liquid Systems in the Shell-and-Tube Heat Exchanger

Two-phase heat transfer involving two immiscible systems is gaining importance in petrochemical and allied industries. Varying compositions of second component (liquid) and water were experimentally studied in the shell side of a 1:2 shell-and-tube heat exchanger. The single-phase heat transfer coefficient on pure water and pure liquid were fitted to the Reynolds number range studied for generating the two-phase parameter correlation. The two-phase multiplier was related to the Lockhart–Martinelli (L-M) parameter using the two-phase experimental data and a correlation was established. The two-phase heat transfer coefficient was calculated based on the coefficients a and m for single-phase data along with two-phase multiplier and L-M parameter. The calculated values of the two-phase heat transfer coefficient based on pure component and pure water suggest that water is a better reference fluid for the two-phase liquid-water systems studied and obtained with an average error ranging between 0.74 and 1.89%. In this experimental work, a general correlation was also developed with respect to dimensionless groups by fitting the two-phase experimental data of seven two-phase systems studied and proved with minimum average absolute deviation between experimental and theoretical values.

Inlet flow effects in micro-channels in the laminar and transitional regimes on single-phase heat transfer coefficients and friction factors

An experimental investigation of heat transfer and pressure drop in rectangular micro-channels was conducted for water in the laminar and transitional regimes for three different inlet configurations. The inlet types under consideration were the sudden contraction, bellmouth, and swirl inlet types, and hydraulic diameters of 0.57, 0.85, and 1.05mm were covered. It was found that the critical Reynolds number and the transitional behaviour in terms of heat transfer coefficients and friction factors were influenced significantly by the inlet type. For the sudden contraction inlet type, which were investigated for both adiabatic, as well as diabatic cases, adiabatic friction factors were predicted well by the laminar Shah and London correlation, while diabatic friction factors were decreased with an increase in wall heat flux. The sudden contraction inlet critical Reynolds numbers were found to be between 1800 and 2000 for adiabatic cases, while for diabatic cases the transition regime commenced at a Reynolds number of about 2000. The bellmouth and swirl inlet types were mostly investigated for diabatic cases only with swirl inlet tests limited to the 1.05mm channel. Laminar friction factors were approximately similar to those of the sudden contraction inlet type, however, after the commencement of transition both inlet types exhibited higher friction factors than the sudden contraction inlet. Minor transition occurred as early as at Reynolds numbers of 1200 and 800 for the bellmouth and swirl inlet types respectively while major transition occurred at Reynolds numbers of approximately 1800 and 1500 respectively. Critical Reynolds numbers were found not be significantly influenced by the channel diameter to length ratio considered in this study. Laminar Nusselt numbers were predicted well by conventional macro-channel thermal entry correlations. The swirl inlet type exhibited the highest friction factors and Nusselt numbers in the transitional regime followed by the bellmouth inlet type. During transition while compared with the sudden contraction inlet, both the bellmouth and swirl inlet types exhibited larger enhancement in heat transfer than increases in the friction factor penalty. Based on the experimental data obtained in this study, a set of correlations were developed which describes the relation between the friction factor and Colburn j-factor. Depending on the inlet type, the correlations predicted between 94% and 100% of the results to within 10% of the experimental measurements.

Experimental study on subcooled flow boiling heat transfer to water–diethylene glycol mixtures as a coolant inside a vertical annulus

Experimental studies on subcooled nucleate flow boiling heat transfer of dilute water–diethylene glycol (DEG) mixtures inside a vertical annulus have been conducted. The experimental apparatus provides the particular conditions to investigate the influence of heat flux, flow velocity, degree of subcooling, and concentration of mixture on heat transfer coefficient in both the convection and nucleate boiling regimes. Results reveal that increasing heat flux causes the single phase heat transfer coefficient increases slightly until the incipience of bubble formation. Then, the flow boiling heat transfer coefficient dramatically increases when heat flux increases. Increasing the flow velocity increases the flow boiling heat transfer coefficient. Contrary to the expectations; little increase in the heat transfer coefficient is clearly observed in both of the convective and the nucleate boiling regimes when small amount of DEG was added to the mixture. In addition, liquid inlet temperature to the annulus significantly influences the inception heat flux such that increasing the inlet temperature decreases the inception heat flux.

Thermally Developing Single-Phase Flows in Microtubes

The pressure drop and heat transfer due to the flow of de-ionized water at high mass fluxes in microtubes of ∼ 254 μm and ∼ 685 μm inner diameters is investigated in the laminar, transition and the turbulent flow regimes. The flow is hydrodynamically fully developed and thermally developing. The experimental friction factors and heat transfer coefficients are respectively predicted to within ±20% and ±30% by existing open literature correlations. Higher single phase heat transfer coefficients were obtained with increasing mass fluxes, which is motivating to operate at high mass fluxes and under thermally developing flow conditions. The transition to turbulent flow and friction factors for both laminar and turbulent conditions were found to be in agreement with existing theory. A reasonable agreement was present between experimental results and theoretical predictions recommended for convective heat transfer in thermally developing flows.

Heat Transfer Characteristics of Nano-Fluids

Nanofluids are engineered colloids made of a base fluid and nanoparticles, which become potential candidate for next generation heat transfer medium. Nanofluids have higher thermal conductivity and single-phase heat transfer coefficients than their base fluids. The use of additives is a technique applied to enhance the heat transfer performance of base fluids. Recent articles address the unique features of nanofluids, such as enhancement of heat transfer, improvement in thermal conductivity, increase in surface volume ratio, Brownian motion, thermophoresis, etc. A complete understanding about the heat transfer enhancement in forced convection in laminar and turbulent flow with nanofluids is necessary for the practical applications. There are many controversies and inconsistencies in reported arguments and experimental results on various thermal characteristics such as effective thermal conductivity, convective heat transfer coefficient and boiling heat transfer rate of nanofluids. As of today, researchers have mostly focused on anomalous thermal conductivity of nanofluids. Although investigations on boiling, droplet spreading, and convective heat transfer are very important in order to exploit nanofluids as the next generation coolants, considerably less efforts have been made on these major features of nanofluids. This review summarizes recent research on fluid flow and heat transfer characteristics of nanofluids in forced and free convection flows and identifies opportunities for future research.

Measurement of subcooled boiling pressure drop and local heat transfer coefficient in horizontal tube under LPLF conditions

Horizontal flow is commonly encountered in boiler tubes, refrigerating equipments and nuclear reactor fuel channels of pressurized heavy water reactors (PHWR). Study of horizontal flow under low pressure and low flow (LPLF) conditions is important in understanding the nuclear core behavior during situations like LOCA (loss of coolant accidents). In the present work, local heat transfer coefficient and pressure drop are measured in a horizontal tube under LPLF conditions of subcooled boiling. Geometrical parameters covered in this study are diameter (5.5mm, 7.5mm and 9.5mm) and length (550mm, 750mm and 1000mm). The operating parameters varied are mass flux (450–935kg/m2s) and inlet subcooling (29°C, 50°C and 70°C). Infra-red thermography is used for the measurement of local wall temperature to estimate the heat transfer coefficient in single phase and two phase flows with water as the working medium at atmospheric pressure. Correlation for single phase diabatic pressure drop ratio (diabatic to adiabatic) as a function of viscosity ratio (wall temperature to fluid temperature) is presented. Correlation for pressure drop under subcooled boiling conditions as a function of Boiling number (Bo) and Jakob number (Ja) is obtained. Correlation for single phase heat transfer coefficient in the thermal developing region is presented as a function of Reynolds number (Re), Prandtl number (Pr) and z/d (ratio of axial length of the test section to diameter). Correlation for two phase heat transfer coefficient under subcooled boiling condition is developed as a function of boiling number (Bo), Jakob number (Ja) and Prandtl number (Pr).

Screening Single Phase Laminar Convective Heat Transfer of Nanofluids in a Micro-tube

Nano scale solid particles dispersed in base fluids are a new class of engineered colloidal solutions called nanofluids. Several studies reported enhancement of heat transfer by using nanofluids. This article reports convective single-phase heat transfer coefficients in an open 30 cm long, 0.50 mm internal diameter stainless steel test section. The setup is used for screening single phase laminar convective heat transfer with water and three different nanofluids: water based Al2O3, ZrO2, and TiO2 (all with 9 wt% of particles). A syringe pump with adjustable pumping speed is used to inject fluids into the test section. Thirteen T-type thermocouples are attached on the outer surface of the test section to record the local wall temperatures. Furthermore, two T-type thermocouples are used to measure inlet and outlet fluid temperatures. A DC power supply is used to heat up the test section and a differential pressure transducer is used to measure the pressure drop across the tube. Furthermore, the effective thermal conductivities of these nanofluids are measured using the Transient Plane Source (TPS) method at a temperature range of 20 – 50°C.The experimental average values of heat transfer coefficients for nanofluids are compared with water. Enhancement in heat transfer of nanofluids is observed only when compared at constant Reynolds number (Due to higher viscosity for nanofluids, higher velocity or mass flow rate is required for nanofluids to reach the same Reynolds number). The other methods of comparison: equal mass flow rate, volume flow rate, pressure drop and pumping power did not show any augmentation of the heat transfer coefficient for the tested nanofluids compared to water.

Two-Phase Heat Transfer and Bubble Characteristics in a Microchannel Array

Heat transfer coefficients and bubble motion characteristics are reported for two-phase water flow in an array of 13 equally spaced microchannels over an area of 1 cm2. Each channel has Dh = 451 ± 38 μm, W/H = 0.8, and L/Dh = 22.2. Uniform heat flux is applied through the base, and wall temperatures are determined from the thermocouple readings corrected for heat conduction effects. The upper surface is insulated and transparent. Single-phase heat transfer coefficients are in a good agreement with comparable trends of existing correlations for developing flow and heat transfer, although a difference is seen due to the insulated upper surface. Two-phase heat transfer coefficients and flow characteristics are determined for 221 &lt; G &lt; 466 kg/m2s and 250 &lt; q &lt; 1780 kW/m2. Heat transfer coefficients normalized with mass flux exhibit a trend comparable to that of available studies that use similar thermal boundary conditions. Flow visualization shows expanding vapor slug flow as the primary flow regime with nucleation and bubbly flow as the precursors. Analysis of bubble dynamics reveals ∼t1/3 dependence for bubble growth. Flow reversal is observed and quantified, and different speeds of the vapor phase fronts are quantified at the leading and trailing edges of vapor slugs once the bubble diameter equals the channel width. Bubble formation, growth, coalescence, and detachment at the outlet of the array are best characterized by the Weber number.

Boiling heat transfer and critical heat flux of ethanol–water mixtures flowing through a diverging microchannel with artificial cavities

This paper presents an experimental study on the convective boiling heat transfer and the critical heat flux (CHF) of ethanol–water mixtures in a diverging microchannel with artificial cavities. The results show that the boiling heat transfer and the CHF are significantly influenced by the molar fraction ( x m ) as well as the mass flux. For the single-phase convection region except for the region near the onset of nucleate boiling with temperature overshoot, the single-phase heat transfer coefficient is independent of the wall superheat and increases with a decrease in the molar fraction. After boiling incipience, the two-phase heat transfer coefficient is much higher than that of single-phase convection. The two-phase heat transfer coefficient shows a maximum in the region of bubbly-elongated slug flow and deceases with a further increase in the wall superheat until approaching a condition of CHF, indicating that the heat transfer is mainly dominated by convective boiling. A flow-pattern-based empirical correlation for the two-phase heat transfer coefficient of the flow boiling of ethanol–water mixtures is developed. The overall mean absolute error of the proposed correlation is 15.5%, and more than 82.5% of the experimental data were predicted within a ±25% error band. The CHF increases from x m = 0–0.1, and then decreases rapidly from x m = 0.1–1 at a given mass flux of 175 kg/m 2 s. The maximum CHF is reached at x m = 0.1 due to the Marangoni effect, indicating that small additions of ethanol into water could significantly increase the CHF. On the other hand, the CHF increases with increasing the mass flux at a given molar fraction of 0.1. Moreover, the experimental CHF results are compared with existing CHF correlations of flow boiling of the mixtures in a microchannel.

사다리꼴 미세유로의 대류비등 2상유동 : 2부-열전달 특성

ABSTRACT:Characteristics of flow boiling heat transfer in microchannels were investigated experimentally. The microchannels consisted of 9 parallel trapezoidal channels with each channel having 205 m of bottom width, 800 m of depth, 3.6 o of sidewall angle, and 7 cm of length. Tests were performed with R113 over a mass velocity range of 150～920 kg/m 2 s, heat flux of 10～100 kW/m 2 and inlet pressures of 105～195 kPa. Flow boiling heat transfer coefficient in microchannels was found to be dominated by heat-flux. However the effect of mass velocity was not significant. Contrary to macrochannel trends, the heat transfer coefficient was shown to decrease with increa-sing thermodynamic equilibrium quality. A new correlation suitable for predicting flow boiling heat transfer coefficient was developed based on the laminar single-phase heat transfer coefficient and the nucleate boiling dominant equation. Comparison with the experimental data showed good agreement.Keywords:Flow boiling heat transfer(유동비등 열전달), Microchannels(미세유로), Quality(건도), Boiling number(비등 수), Martinelli parameter(마티넬리 변수)

Refrigerant flow boiling heat transfer in parallel microchannels as a function of local vapor quality

Flow boiling of refrigerant HFC-134a in a multi-microchannel copper cold plate evaporator is investigated. The heat transfer coefficient is measured locally for the entire range of vapor qualities starting from subcooled liquid to superheated vapor. The test piece contains 17 parallel, rectangular microchannels (0.762 mm wide) of hydraulic diameter 1.09 mm and aspect ratio 2.5. The design of the test facility is validated by a robust energy balance as well as a comparison of single-phase heat transfer coefficients with results from the literature. Results are presented for four different mass fluxes of 20.3, 40.5, 60.8, and 81.0 kg m −2 s −1, which correspond to refrigerant mass flow rates of 0.5–2.0 g s −1, and at three different pressures 400, 550 and 750 kPa corresponding to saturation temperatures of 8.9, 18.7, and 29 °C. The wall heat flux varies from 0 to 20 W/cm 2 in the experiments. The heat transfer coefficient is found to vary significantly with refrigerant inlet quality and mass flow rate, but only slightly with saturation pressure for the range of values investigated. The peak heat transfer coefficient is observed for a vapor quality of approximately 20%.

Single-phase and two-phase cooling using hybrid micro-channel/slot-jet module

This paper explores the single-phase and two-phase cooling performance of a hybrid micro-channel/slot-jet module using HFE-7100 as working fluid. Three-dimensional numerical simulation using the k–ε turbulent model is used to both assess the single-phase performance and seek a geometry that enhances heat removal capability and surface temperature uniformity while decreasing mean surface temperature. This geometry is then tested experimentally to validate the numerical findings and aid in the development of correlations for both the single-phase and two-phase heat transfer coefficients. The hybrid module is shown to maintain surface temperature gradients below 2°C for heat fluxes up to 50W/cm2. Even without phase change, the hybrid module is capable of dissipating heat fluxes as high as 305.9W/cm2. Highly accurate single-phase correlations are developed using a superpositioning technique that consists of assigning a different heat transfer coefficient for each portion of the heat transfer area based on the dominant heat transfer mechanism for that portion. Increasing subcooling and/or flow rate is shown to delay the onset of nucleate boiling to a higher heat flux and higher surface temperature, as well as enhance critical heat flux (CHF). A correlation previously developed for hybrid micro-channel/micro-circular-jet module is deemed equally effective at predicting two-phase heat transfer data for the present hybrid module.

A minichannel aluminium tube heat exchanger – Part I: Evaluation of single-phase heat transfer coefficients by the Wilson plot method

A prototype liquid-to-refrigerant heat exchanger was developed with the aim of minimizing the refrigerant charge in small systems. To allow correct calculation of the refrigerant side heat transfer, the heat exchanger was first tested for liquid-to-liquid (water-to-water) operation in order to determine the single-phase heat transfer performance. These single-phase tests are reported in this paper. The heat exchanger was made from extruded multiport aluminium tubes and was designed similar to a shell-and-tube heat exchanger. The heat transfer areas of the shell-side and tube-side were approximately 0.82 m 2 and 0.78 m 2, respectively. There were six rectangular-shaped parallel channels in a tube. The hydraulic diameter of the tube-side was 1.42 mm and of the shell-side 3.62 mm. Tests were conducted with varying water flow rates, temperature levels and heat fluxes on both the tube and shell sides at Reynolds numbers of approximately 170–6000 on the tube-side and 1000–5000 on the shell-side, respectively. The Wilson plot method was employed to investigate the heat transfer on both the shell and tube sides. In the Reynolds number range of 2300–6000, it was found that the Nusselt numbers agreed with those predicted by the Gnielinski correlation within ±5% accuracy. In the Reynolds number range of 170–1200 the Nusselt numbers gradually increased from 2.1 to 3.7. None of the previously reported correlations for laminar flow predicted the Nusselt numbers well in this range. The shell-side Nusselt numbers were found to be considerably higher than those predicted by correlations from the literature.

The Nukiyama curve in water spray cooling: Its derivation from temperature–time histories and its dependence on the quantities that characterize drop impact

Heat transfer from hot aluminium walls to cold water sprays was investigated. The method used was the transient two-side symmetric cooling of a planar aluminium target, previously heated to temperatures of up to 750 K, by twin sprays issuing from full-cone swirl spray nozzles of various gauge. The target’s mid-plane temperature was recorded during the cooling transient by thin-foil K thermocouples and a high-frequency data acquisition system. In order to determine the wall temperature T w, the wall heat flux q w ″ and the q w ″ - T w heat transfer (Nukiyama) curve, two different approaches were used: the first was based on the solution of an inverse heat conduction problem, the second on a suitable parameterization of the Nukiyama curve and on the solution of a minimum problem. Relevant heat transfer quantities, such as the critical heat flux and the single-phase heat transfer coefficient, were obtained from each heat transfer curve. Their dependence on the main parameters characterizing the spray impact phenomenon (mass flow rate, drop velocity and drop diameter) was investigated on the basis of a preliminary hydrodynamic characterization study, and suitable correlations were proposed.