Subcooled Flow Boiling Heat Transfer Research Articles

Investigating subcooled flow boiling heat transfer and fluid dynamics within a shell-and-plate heat exchanger

The study experimentally investigates subcooled flow boiling in a single channel of a shell-and-plate heat exchanger. Experiments on single-phase and boiling heat transfer were conducted under varying mass flux (61.442 to 199.326 kg/m2·s), preheating (20 to 42℃), and heating temperatures (30 to 60℃), using the refrigerant R365mfc. Key findings include that microscale boiling is the dominant heat transfer mechanism, with macroscale boiling occurring at high mass flux and preheating temperatures. The boiling heat transfer coefficient and friction pressure drop increase with mass flux, while optimal heat transfer occurs with an inlet subcooling of 4–6 °C. Flow patterns—bubbly, bubbly-slug, and churn flows—were identified, with bubbly flow offering superior heat transfer performance at the cost of higher pressure drop. The prediction model for friction pressure drop and heat transfer coefficient, using the Weber and boiling numbers, shows a prediction error within ±20 % of experimental data.

Comparison of subcooled flow boiling in concentric annuli with bare and finned surfaces

In this study, the hydrodynamic and thermal characteristics of subcooled flow boiling in concentric annuli with inner heating wires, featuring bare and finned surfaces designed for heat transfer enhancement, were experimentally investigate, with a focus on cooling technology for EV charging cables. The bare wire test module consisted of a 300 mm long wire, with a 290 mm long inner heating surface and a diameter of 10 mm. The outer tube was constructed from transparent polycarbonate with a 22 mm diameter, allowing optical access to observe the flow. The finned wire test module has an identical construction, except for the addition of six longitudinally extruded fins with cross-sectional dimensions of 2 × 5 mm2. Experiments and flow visualizations, aided by high speed imaging, were conducted using HFE-7100 as the working fluid at mass velocities, G, 463.7 to 1,309.6 kg/m2s and heat fluxes from 0.90 to 9.93 W/cm2, under inlet pressures between 120 and 240 kPa. The study investigated thermal non-equilibrium phenomena, driven by asymmetric bubble accumulation, and their effects on circumferential temperature distribution, which showed opposing trends between the bare and finned surfaces. An evaluation of previous empirical correlations for predicting subcooled flow boiling heat transfer coefficients was performed, followed by an assessment of fin performance in enhancing heat transfer. A comparative analysis with the bare surface module of the same dimensions confirmed that the longitudinal fins provided superior thermal management, maintaining the surface temperature below the safety threshold, Ts,safe. The longitudinal fins demonstrated a 54.5 % reduction in charging time and a 183.7 % increase in charging current for charging up to 80 % state of charge (SOC) of a 77.4 kWh battery, compared to the bare wire.

The interaction of surface orientation and roughness on nanofluid sub-cooled flow boiling

This experimental study examined the subcooled-flow boiling of water-alumina nanofluids with 0.1 vol % concentration. The study aimed to explore how surface roughness, fluid velocity, and surface orientation influenced the subcooled flow boiling heat transfer (SFBHT). A cross-sectional area of 2 × 3 cm2 and a length of 1.20 m are the dimensions of the plexiglass channel in the experimental setup. A copper heater was embedded at the bottom surface of the channel. The results of the experiment revealed that the surface roughness had a positive effect on SFBHT for horizontal channels, and it depended on the surface configuration. The heat flux increases by 233 % and 98.27 % at low and high fluid velocity (0.5 and 0.9 m/s) by changing the surface roughness from 0.65 to 4.4 μm, respectively. The fluid velocity did not have uniform impacts on SFBHT for higher and lower surface temperatures. When altering the fluid velocity from 0.5 m/s to 0.9 m/s, the heat flux experiences significant variations: The overall heat fluxes are 56.52 % and −35.86 % for rough surface at lower and higher surface temperature, respectively. Depending on whether the surfaces were smooth or rough, the SFBHT performance varied due to the surface configuration. The boiling heat transfer (BHT) performance increased for smooth surfaces but decreased for rough surfaces by changing the surface orientation in both negative and positive directions. The overall heat fluxes increase by about 72.84 % and 66.67 % at smooth surface for both positive and negative inclination, respectively. They are about −32.94 % and −29.82 % at rough surface for both positive and negative inclination, too. The rate of surface roughness’ effects on SFBHT reduces by enhancing the surface angles.

Heat transfer analysis of subcooled flow boiling in copper foam helical coiled heat exchanger – A pore-scale numerical study

High performance thermal energy systems are crucial in fulfilling the growing demand of thermal energy. Consequently, numerous heat transfer enhancements have been proposed. Some of the promising methods are boiling flow heat transfer, helical coiled tube and metal foam insert. Although the performance of each strategy has been extensively studied, based on our literature review, the effectiveness of combining these strategies has not been investigated. This missing information may impede further development of efficient heat transfer systems. This study is therefore performed to investigate subcooled flow boiling heat transfer in helical tube with copper foam insert. A high-resolution three-dimensional pore scale model that considers the shape, porosity and pore density of the metal foam is developed and validated against the published experimental results. Using the model, the effect of inlet temperature, mass flux, wall heat flux, porosity, pore density and half-filled metal foam are studied. The results suggest that operating parameters have mild effects on the heat transfer performance. In contrast, porosity and pore density show significant influence in heat transfer coefficient and pressure drop. It has also been found that total heat transfer surface area and base area are better in characterizing heat transfer in pore-scale level than porosity and pore density at pore-scale level. Moreover, implementing metal foam at outer half of the tube is observed to offer slightly better heat transfer than implementing it at the inner side by around 4.8%. Overall, this study indicates that helical metal foam tubes offer the best performance at mass flux of 200 kg/m2∙s with performance index of 0.72, while straight tubes with 0.90 porosity has the highest performance index of 0.66. Empirical correlations to relate dimensionless parameters to the Nusselt number and friction factor is generated. Finally, the model developed in this study can serve to provide guidelines for the upcoming studies in boiling in metal foam structure and assist in designing metal foam heat exchangers.

An improved wall boiling model for numerical simulation of subcooled flow boiling on a new hybrid micro/nanostructured surface

Previous studies have demonstrated that micro/nanostructured surfaces have great potential for heat transfer enhancement. However, simulating subcooled flow boiling on such surfaces is difficult owing to the lack of proper bubble characteristic parameter models, because most models used in flow boiling simulations were developed based on smooth surface conditions, which may limit their applications in engineering design. In this study, we improved upon one validated bubble characteristic parameter model suitable for subcooled flow boiling on smooth surfaces to adapt to the new hybrid micro/nanostructured surfaces proposed by Huang et al. [“Experimental investigation of a new hybrid structured surface for subcooled flow boiling heat transfer enhancement,” Appl. Therm. Eng. 192, 116929 (2021)]. The new bubble characteristic parameter model incorporates both basic correction terms to account for boiling bubble behaviors and ad hoc parameters to account for other unknown effects. Through sensitivity analysis and detailed calibration, the model was simplified to a set of correlations and only one constant parameter. With this improved model, subcooled flow boiling heat transfer simulations were conducted for three target surface specifications under conditions of 4–10 MW/m2 incident heat flux and 1–5 m/s flow velocity and the related heat-transfer mechanisms were further compared and discussed. The maximum error between the simulation and experimental results remains less than 3.5%, indicating that the established model has considerable accuracy in predicting the heat transfer performance for this type of micro/nanostructured surface in high-heat-flux engineering design applications. The heat transfer enhancement for subcooled flow boiling on this type of hybrid micro/nanostructured surface is greatly beneficial owing to its proper organization of convection, evaporation, and quenching heat transfer.

New correlation of the subcooled flow boiling heat transfer coefficient for hybrid micro/nanostructured surfaces with high heat flux incidence

Surfaces manipulated with micro/nanostructures have great potential for imparting high heat removal capacity to heat transfer equipment under high heat flux incidence. However, few correlations of heat transfer coefficients have been developed based on high-heat flux experiments of subcooled flow boiling with nano- and microstructured surfaces, which may limit the further application of this technique in industrial engineering. Aiming at recently developed hybrid micro/nanostructured surfaces for subcooled flow boiling performance enhancement, a series of experiments with different surface structure scale specifications, flow conditions, and heat flux incidence ranges were conducted. In total, 530 experimental data points were obtained with heat fluxes ranging from 3.4 to 13.1 MW/m2, pressures from 0.5 to 1.5 MPa, liquid velocities from 1 to 5 m/s, inlet temperatures from 303.15 to 323.15 K, and structured surfaces with wire apertures of 250, 125, and 58 μm. The 28 existing correlations based on smooth surfaces and structured surfaces failed to predict these experimental data. A new enhancement factor-type correlation model, which included four dimensionless numbers that specialized in the effects of the span dimension and depth dimension of the target hybrid micro/nanostructured surfaces, was developed and shown to have good prediction accuracy and a considerable degree of application adaptability, with a mean absolute deviation of 5.7%.

Thermo-hydraulic performance of tungsten flat tile type divertor target with two types of novel engineered cooling channel wall

The divertor is one of the key components in magnetic confinement fusion reactors. Improving the heat transfer performance of the divertor has great importance for promoting its service capacity. This work proposed a new kind of cooling surface configuration for a flat-type divertor design, which is based on the concept of synergetic heat transfer enhancement via combination of a sinusoidal undulation structure with a micro-nano structured surface (copper-mesh brazed coating). The combination surface configuration is preliminarily obtained by numerical optimization based on the calibrated model of subcooled flow boiling heat transfer. Then, experimental mockups were fabricated and tests with high heat flux (HHF) conditions were conducted to examine the effects of the new design. Both the numerical and experimental results confirm that the incident critical heat flux is increased from 15 MW/m2 of the micro-nano structured flat surface case to over 22 MW/m2 of the combination case under the conditions of relatively low flow velocity of 3 m/s and low flow resistance increase of 7.3 %.

Determining of bubble waiting time coefficient, critical Webber number and turbulence model for modeling of subcooled flow boiling heat transfer of CO2 at low temperature

Determining of bubble waiting time coefficient, critical Webber number and turbulence model for modeling of subcooled flow boiling heat transfer of CO2 at low temperature

A numerical investigation of subcooled flow boiling heat transfer in twisted elliptical tubes

Twisted elliptical tubes possess superior heat transfer performance due to their unique geometry. It is necessary to further explore their potential applications in high-efficiency heat transfer devices. This study conducted a numerical simulation investigation to analyze the flow boiling heat transfer characteristics of subcooled water inside twisted elliptical tubes. The results show that the spiral flow inside twisted elliptical tube causes the fluid with relatively small vapor volume fraction to tend to distribute near the wall, while also increases turbulence intensity and promotes interactions among bubbles. Moreover, increasing the aspect ratio of the twisted elliptical tube can effectively improve the heat transfer coefficient and has a relatively small impact on the pressure drop of the fluid in the tube. Simulation data show that when the aspect ratio increases from 1.41 to 1.80, the heat transfer coefficient of the pipe increases by 15.21 %, while the pressure drop increases by 3.34 %.

Heat transfer distribution and pressure drop fluctuations in subcooled flow boiling at subatmospheric system pressure

The transfer of higher heat flux at subatmospheric system pressure is relevant to different applications where the higher surface temperature is a constraint. There is a scarcity of literature on subcooled flow boiling at subatmospheric system pressure. The experiments are performed at 50, 75 and 101 kPa absolute system pressure in an SS-304 tube of 5.85 mm radius, heated length of 1500 mm and 0.5 mm thickness. The mass flux of 90–300 kg/m2s is streamed at the varying heat flux of 48–218 kW/m2. The heat transfer coefficient in single-phase flow is not impacted significantly by the subatmospheric system pressure. The subcooled flow boiling heat transfer coefficient is increased at the lower subatmospheric system pressure. The more nucleation sites due to higher wall superheat are activated at the higher heat flux at subatmospheric system pressure. The surface temperature is radially symmetric at subatmospheric system pressure for all values of the Froude number. The gravitational force is suppressed at subatmospheric system pressure. The onset of nucleation is achieved earlier in the axial direction at the higher heat flux. The mass flux has no significant effect on the subcooled flow boiling heat transfer coefficient. The pressure drop and fluctuations are enhanced at the higher mass flux, higher heat flux and lower subatmospheric system pressure.

Irregular-shaped nucleated bubbles induced enhancement of subcooled flow boiling heat transfer in leaf vein inspired three-tiered open microchannels (LTOMCs)

Nature is an important source of inspiration for developing desired structures for various industrial applications. The efficacy of water transport in leaf vein systems provides important insights for the structural design of microchannels. In addition, open microchannels (OMCs) have been reported to effectively reduce the two-phase pressure drop and suppress flow instabilities by providing extra space for bubble and vapor expansion. Thus, the synergistic cooperation of open microchannels with leaf-vein-like structures is expected to alter the flow patterns and improve the heat transfer performance in the flow boiling process. In this work, two leaf-vein inspired three-tiered open microchannels (LTOMC) with exactly the same structure but opposite orientations of vein-like channels, i.e., LTOMC-Ⅰ and LTOMC-Ⅱ, were manufactured, respectively. Flow boiling experiments were conducted at the inlet temperature of 25 °C under different mass fluxes. It was observed that the flow patterns of the LTOMC were divided into bubbly flow, slug flow, reverse and rewet flow; the shapes and movement trends of nucleated bubbles were mainly dominated by the vein-like structures. The required wall superheat of the onset of nucleated boiling for LTOMCs was 2–6 °C lower than ROMC; the one for LTOMC-Ⅱ was 2–3 °C lower than LTOMC-Ⅰ at various mass fluxes. The average two-phase heat transfer coefficient of LTOMC-Ⅱ was increased by 83.2% compared to ROMC at the mass flux of 234 kg/m2·s; the one of LTOMC-Ⅱ was 54.1% higher than LTOMC-Ⅰ. The pressure drop of three OMCs was comparable; however, the onset of two-phase flow instability occurred earlier for ROMC at the same heat flux. Besides, the liquid rewetting capability of ROMC was easier to get deteriorated at ultra-high heat flux. This work offers an avenue for structural optimization of microchannel heat sinks to enhance heat transfer performance without compromising the pressure loss.

Numerical investigation of electrohydrodynamically enhanced flow boiling inside minichannels: The seeding model

In the present study, three-dimensional numerical simulations are performed to examine the effect of applied direct current electric fields on the subcooled flow boiling heat transfer in a vertical minichannel. The volume of fluid model is used to capture the liquid–vapor interfaces, and the conservation laws, together with the electrohydrodynamic (EHD) equations, are solved using the finite volume method. Two mass transfer models of Lee and Fourier are evaluated, and a new seeding algorithm is developed based on the physics of the bubble formation and departure on the heated walls, integrated with the Fourier model. The early transition from slug to churn/annular flow regimes, due to EHD forces, is observed in the numerical solutions with both models, in agreement with the available experiment. Nevertheless, the Lee model tuned for the electric-free condition fails to predict the EHD-induced heat transfer enhancement, while the Fourier model with the new seeding algorithm captures this phenomenon with reasonable accuracy. Based on the present numerical results, this effect can be attributed to the migration of small bubbles toward the walls under the effect of electric fields, forming recirculating flow regions near the walls, augmenting the flow mixing, and mitigating hot spots.

Investigation and measurement of crude oil heat transfer coefficient in forced convection and subcooled flow boiling heat transfer

Investigation and measurement of crude oil heat transfer coefficient in forced convection and subcooled flow boiling heat transfer

Experimental study on subcooled flow boiling heat transfer of water-SDS mixtures in a vertical macrochannel

Flow boiling heat transfer performance of surfactant solutions is studied at a basis of deionized water. The tested fluids are aqueous solutions of SDS at concentrations ranging from 100 to 12500 ppm. Tests are performed at vertical, upward, 3 × 40 mm cross-section channel at 75 °C subcooling, atmospheric pressure, flow rates 200–300 kg/m2s (Re: 1400–2600) and heat fluxes 200–1000 kW/m2. The present experimental data show that the trend of heat transfer enhancement follows the opposite direction of surface tension trend when surfactant concentration increases. At CMC (Critical Micellar Concentration) heat transfer coefficient and bubble density take their maximum value, while bubble diameter takes its minimum value. Concentrations above CMC show minimal or no further effect on flow boiling heat transfer characteristics. The tendency of the heat transfer coefficient experimental data with respect to changes in surfactant concertation allows the development of a modified version of the well-known two phase model heat transfer of Shah, by incorporating a surfactant concentration dependent correction factor that predicts this work’s values at a ±10% margin and a mean absolute percentage error of 3.6%.

Modeling and study of microlayer effects on flow boiling in a mini-channel

The microlayer evaporation below the vapor bubble contributes much to heat dissipation and bubble growth, but it is hard to explore this microcosmic phenomenon. This study proposes a method for investigating the subcooled flow boiling heat transfer in a rectangular channel by considering the microlayer, conjugated heat transfer, a reasonable nucleation site density, and an accurate liquid-vapor interface. The implementation procedure of the microlayer model in the numerical simulation is fully demonstrated. Based on that, the bubble growth, flow pattern, heat transfer, microlayer depletion, evolutions of evaporative heat flux and dry patch during the subcooled flow boiling under different initial microlayer thicknesses are presented. Results indicate that microlayer evaporation contributes much to bubble volume and heat dissipation. The vapor volume generated by the microlayer evaporation exceeds that flows out of the channel because of the condensation effect at the liquid-vapor interface induced by the subcooled liquid, indicating bubble growth and expansion are completely attributed to microlayer evaporation. Simultaneously, much heat dissipates from the heat source due to microlayer evaporation, which contributes over 70% to both local and whole heat transfer. Because of microlayer depletion, some dry patches may appear at the elongated bubble tail. Therefore, liquid slug, elongated bubble, and dry patch circulate in the studied mini-channel, corresponding to heat transfer modes of liquid convection, microlayer evaporation, and vapor convection, supporting the three-zone heat transfer model. In addition, the initial microlayer thickness is related to the thermal resistance and affects the microlayer evaporation rate, leading to different flow patterns and heat transfer characteristics. The initial microlayer thickness is related to the slipping velocity of bubbles based on the Taylor model, which is recommended for the flow boiling study in a mini-channel.

A novel integrated PDB-FDB model for the prediction of flow boiling heat transfer under high sub-cooling and very high heat flux conditions

Although numerous studies on subcooled flow boiling heat transfer have been conducted, those evaluating very high heat fluxes of several MW/m2 under one-side heating conditions are limited. In this study, the heat transfer characteristics of a circular smooth channel are evaluated under one-sided heating conditions with a high heat flux of up to 11.82 MW/m2. An experiment to measure the heat transfer (including boiling performance) from single-phase to critical heat flux (CHF) under an inlet pressure of 0.1–1 MPa, mass flux of 2000–6000 kg/m2s, and inlet bulk temperature conditions of 30–140 °C is conducted. The boiling regime is analyzed by dividing it into a partially developed nucleate boiling (PDB) regime and fully developed nucleate boiling (FDB) regime, using the onset of the nucleate boiling (ONB) point and onset of the fully developed nucleate boiling (OFDB) point. A total of 15 existing subcooled flow boiling heat transfer correlations for the PDB and FDB regimes are evaluated with the experimental results. In the PDB regime, the Liu and Winterton correlation reveal good predictability within 11%. However, all the evaluated existing correlations of the FDB regime exhibit high error rates. In this study, a new FDB correlation is developed by considering the agitation heat flux (an expected heat transfer characteristic in the FDB regime) in the Liu and Winterton correlation with Python code and the AI regression method. An evaluation of the developed correlation with the data of other previous studies, such as those of Araki et al. and Jianguo Yan et al. reveals that the newly proposed correlation can predict the FDB heat transfer under conditions of higher pressure (approximately 5 MPa) and higher flow rate (approximately 10,000 kg/m2s) with a low error rate of 17%.

Air rumbling for online fouling mitigation of calcium carbonate aqueous solution under subcooled flow boiling heat transfer

In this research, increasing the shear stress by injecting air into the working system was implemented to mitigate crystallization fouling of calcium carbonate (CaCO3) aqueous solution. For this purpose, various experiments were conducted under subcooled flow boiling condition for analysing the effect of air injection on reduction of fouling formation inside a vertical annular channel with upward flow. These experiments were conducted at different operating conditions including air injection rate of 2–10 l/min, air injection freqency (f) of 0.5–2 h−1, heat flux of 52–83 kW/m2, fluid flow rate of 2–4 l/min, fluid bulk temperature (Tb) of 30–55 °C, and at a constant CaCO3 concentration of 0.2 g/l. The results showed that by increasing f from 0.5 to 2 h−1, the rate of fouling formation decreased. Also, the air rubmbling was more effective when Tb increased from 30 °C to 55 °C, by decreasing the fouling resistance from 38% to 57% comparing with no-mitigation experiment. When the heat flux was 83 kW/m2, air rumbling could decrease the fouling resistance by 57%. On the other hand, at lower heat flux of 52 kW/m2, air injection not only failed to reduce the fouling resistance, but increased it by about 20%. Also, when the air injection rate increased from 2 to 10 l/min, fouling resistance increased slightly. This environmentally friendly method has not received much attention before. Hopefully, the results presented in this research can pave the way for more research works on the mitigation of crystallization fouling using air injection.

Subcooled flow boiling heat transfer in a partially-heated rectangular channel at different orientations in Earth gravity

This study explores subcooled flow boiling of n-Perfluorohexane in a rectangular channel of 5.0 mm height, 2.5 mm width (heated), and 114.6 mm heated length. This Flow Boiling Module (FBM) is part of the Flow Boiling and Condensation Experiment (FBCE), which is a long-term collaborative effort to study the effects of gravity on flow boiling and condensation for their implementation in future space missions besides other applications. Datasets obtained from subcooled-inlet experiments performed using the FBM prior to its launch to the International Space Station (ISS) are examined and local subcooled flow boiling datapoints compiled to form a consolidated database. The consolidated 2589 datapoints cover broad ranges of operating conditions (mass velocity: 172.79 – 3200.00 kg/m2s, pressure: 102.16 – 238.44 kPa, subcooling: 0.13 – 34.93°C, quality: -0.560 – -0.000, heat flux: 1.80 – 49.99 W/cm2), flow orientations (vertical upflow, vertical downflow, and horizontal flow in Earth gravity), and partial heating configurations (single- and double-sided heating). Prior seminal correlations for subcooled flow boiling heat transfer are assessed for their predictive performance. Each local flow boiling curve is analyzed and manually demarcated into three regimes: partially developed boiling (PDB), fully developed boiling (FDB), and nucleate boiling degradation (NBD). A simple, yet very effective correlation based on dimensionless groups and having a fully explicit functional form, is developed. The new correlation well predicts both the subcooled flow PDB and FDB regimes with overall mean absolute errors (MAEs) of 9.59% and 6.91%, respectively. Although predictions for the NBD regime are much higher than observed with an overall MAE of 41.71%, they can be treated as upper limits to heat transfer here. The correlation is independent of both flow orientation and heating configuration. Overall, the new correlation clearly outperforms all prior seminal correlations for the entire consolidated database, with excellent predictions for both partially and fully developed subcooled flow boiling, i.e., for heat fluxes ranging from onset of nucleate boiling to onset of nucleate boiling degradation.

Experimental study of the effect of metal foams on subcooled flow boiling heat transfer of water and developing a correlation for predicting heat transfer

This paper has investigated the effect of different porosities (0.80–0.90) of the metal foam pipes on heat transfer coefficient and pressure drop of subcooled flow boiling of water experimentally. The metal foam pipe with 0.80 porosity has increased the Nusselt number and pressure drop by 41 % and 21 % compared to the 0.90 metal foam pipe, respectively. The heat transfer of water phase change in a porous medium has received less attention due to the complexities of the problem. One of the challenges in this field is the absence of a correlation for predicting heat transfer in metal foams. In this paper, a systematic method based on dimensionless numbers considering the essential parameters, like mass flux, subcooling, and heat flux, was used to provide a correlation for calculating the Nu-number based on 106 experimental points. The mean absolute deviation (MAD) of the results predicted by the new correlation is 8.9 %, and it predicts 95 % of the database with ±20 %.

Trefftz-Type Functions Applied for Subcooled Flow Boiling Heat Transfer Calculations in a Minichannel of Different Spatial Orientation

ABSTRACT The paper describes the results of subcooled flow boiling heat transfer in an asymmetrically heated group of five minichannels 1 mm deep. The heated element for the cooling fluid (FC-72) flowing in the minichannels was a foil made of Haynes-230 alloy. The time-dependent temperature distribution of the outer heated foil surface was recorded as a result of infrared thermography. The test section was oriented in several spatial orientations, that is, 30°, 45°, 60° and 75° inclinations to the horizontal plane. The objective of the numerical calculations, referring to the experimental results, was to determine the time–dependent heat transfer coefficient on the wall-fluid contact surface from the Robin boundary condition. Both the foil and the fluid temperatures were the result of solving the inverse and direct two-dimensional heat transfer problem in two neighboring domains: the heated foil and flowing fluid. The problem was solved using the Trefftz method and the FEM with Trefftz type basis functions. The heat transfer coefficient obtained by two numerical methods appears to be similar. During subcooled flow boiling, at higher minichannel inclination angles, higher values of the heated foil temperature were obtained, as well as higher values of the heat transfer coefficient.