Values Of Heat Transfer Coefficient Research Articles

МЕТОДИКА ЭКСПЕРИМЕНТАЛЬНОГО ИЗУЧЕНИЯ ПРОЦЕССА ТЕПЛООБМЕНА С ИСПОЛЬЗОВАНИЕМ ИМИТАЦИОННЫХ МОДЕЛЕЙ В РЕАЛЬНОМ ВРЕМЕНИ

The laboratory installation “Study of heat transfer in a pipe-in-pipe heat exchanger with a storage water heater” was supplemented with a digital component that meets the modern level of information technology. The laboratory bench was equipped with new digital temperature and flow sensors. Temperature sensors are installed at the inlet and outlet of coolants. An arduino uno controller is installed to collect data from sensors, perform primary processing and send information to the computer. To visualize sensors data, a program was written in the julia programming language. The program displays 6 figure. Two figures demonstrate the change in the quantities measured by the sensors (temperatures and flow rates) over time. The remaining figures show the calculated values. Based on temperatures and flow rates of coolant, the change in heat flow over time is calculated. Based on the heat flow and the average driving force, the heat transfer coefficient is calculated, calculated from the thermal loads calculated from the hot and cold coolant. Based on a large number of experiments, a criterion equation for calculating heat transfer coefficients for hot and cold coolants was obtained. The change in heat transfer coefficients over time is also demonstrated in program. To understand the processes occurring inside the heat exchanger, the temperature change along the length of the heat exchanger is also shown on the screen. Temperature profiles are calculated based on solving the differential equation for the plug-flow model. The heat transfer coefficient values are calculated using the founded criterion equations. Upon completion of the experiment, the accumulated data is sent to the web server and it is available to students via a link that is shown as a qr code in the program window. The web server determines the areas of stationarity and processes the results according to the methodological instructions for the students. The results of processing are available to the professor and facilitate the search for errors and inaccuracies in the calculations carried out by the student.

A COMBINED ACTIVE (PIEZOS) AND PASSIVE (MICROSTRUCTURING) PARTIAL FLOW-BOILING APPROACH FOR STABLE HIGH HEAT-FLUX COOLING WITH DIELECTRIC FLUIDS

Controlled but explosive growth in vaporization rates is made feasible by ultrasonic acoustothermal heating of the microlayers associated with microscale nucleating bubbles within the microstructured boiling surface/region of a millimeter-scale heat exchanger (HX). The HX is 5 cm long and has a 1 cm × 5 mm rectangular cross section that uses saturated partial flow-boiling operations of HFE-7000. Experiments use layers of woven copper mesh to form a microstructured boiling surface/region and its nano/microscale amplitude ultrasonic (~1-6 MHz) and sonic (< 2 kHz, typically) vibrations induced by a pair of very thin ultrasonic piezoelectric-transducers (termed piezos) that are placed and actuated from outside the heat-sink. The ultrasonic frequencies are for substructural microvibrations whereas the lower sonic frequencies are for suitable resonant structural microvibrations that assist in bubble removal and liquid filling processes. The flow and the piezos' actuation control allow an approximately 5-fold increase in heat transfer coefficient value, going from about 9000 W/m<sup>2</sup>-°C associated with microstructured no-piezos cases to 50,000 W/ m<sup>2</sup>-°C at a representative heat flux of about 25 W/cm<sup>2</sup>. The partial boiling approach is enabled by one inlet and two exit ports. Further, significant increases to current critical heat flux values (~70 W/cm<sup>2</sup>) are possible and are being reported elsewhere. The electrical energy consumed (in W) for generating nano/micrometer amplitude vibrations is a small percentage (currently < 3%, eventually < 1% by design) of the total heat removed (in W), which is a heat removal rate of 125 W for the case reported here.

CFD Analysis for the Variation of Velocity inside Different Types of Ribs

Abstract: Heat exchanger is used to accomplish the industrial demand. Heat Exchanger are necessary for nuclear reactors and in chemical engineering, for of large quantity of heat is conveying in a slight space with high heat transmission rates and slight habitation time distributions even it suffers through a disadvantage of larger pressure drop. From my analysis, it is found that the uses of ribs inside the solar heat exchanger increase the heat transfer with increase in Reynolds number. Through analysis, it is also found that the value of heat transfer coefficient increases with increase in Reynolds number, heat transfer coefficient is maximum for rectangular shape rib in both the cases that are during using water and nano fluid as a working fluid. The average heat transfer coefficient ratio graph shows that rib used in the solar heat exchanger with nano fluid flow shows more heat transfer enhancement as compared to a solar heat exchanger with simple water flow. Overall it is found that rectangular shape ribs show more heat transfer coefficient as compared to other rib.

An Investigation into The Enhancement of Heat Transfer in Roughened Ducts of Solar Air Heaters

One of the most crucial tools for the process of transforming solar energy into thermal energy is a solar air heater. Thanks to its low cost and ease of installation, solar air heaters have quickly become one of the most popular and widely used methods of harvesting solar energy. Low convective heat transfer coefficient values between the absorber plate and the air significantly reduce the solar air heater's thermal efficiency. This is because absorber plates are used in solar air heaters. As a consequence, the absorber plate heats up, releasing a great deal of thermal energy into the surrounding space. This article presents the findings of a study that used computational fluid dynamics to investigate how heat is transferred in a solar air heater. The work for this project was done by the author (CFD). Researchers are now investigating the impact of the Re on the Nu. Commercial software known as ANSYS FLUENT 20 may be used to analyse and visualise the flow that happens across the duct of a solar air heater. This programme falls under the category of finite volume software. Using the programme helps get the job done.

The effect of outer container geometry on the thermal management of lithium-ion batteries with a combination of phase change material and metal foam

The use of PCM for thermal management of the batteries has an important place among passive cooling methods because it does not require pump or fan power. In battery cooling applications with PCM, it was aimed to increase the thermal conductivity of the PCM and to cool the battery better by using fins, nanoparticles, and metal foam. The novelty of this study is to investigate the effect of outer container geometry on battery temperature in battery cooling with PCM + metal foam composition. In this direction, five different container geometries affecting the heat transfer by conduction and convection were analyzed. The parametric study was carried out for different values of convection heat transfer coefficient, heat generation in the battery, PCM amount, and porosity. The lowest battery temperature for a given discharge time was obtained in triangular container geometry. With the use of a triangular container, the battery temperature was reduced between 0.4 % and 14.42 % compared to the use of a circular container, depending on the parameter values. The analysis findings revealed that the outer container geometry is more effective in the thermal management of the battery for conditions with low convection heat transfer coefficient and high heat generation.

Modeling of thermal processes when melting ice on power lines

An analysis of the problem of ice formation on power line wires was carried out, and a review of work in the field of melting ice on various lines was carried out. Existing achievements and problems are noted. One of the reasons for the low equipment of lines with automated melting systems is the need to transfer the line from the operating state to the melting mode. One way to solve the problem is to combine these modes by loading the line with additional current without increasing the power transmitted to consumers. A set of issues on modeling thermal processes in the ice shell of power line wires during ice melting is considered. Limiting the power of heat generation in wires leads to the need to build a sufficiently accurate mathematical model to guarantee melting at different values of wind speed and temperature. A study of wire heating processes was carried out using models of stationary and non-stationary thermal conductivity. An algorithm is proposed for solving the thermal problem when a heated wire penetrates an ice shell, based on simulating the movement of the interface between the solid and liquid phases of water. The features of modeling the melting and crystallization process using a numerical method are described. When ice transitions to a molten state, melting reflects a change in density and the formation of an air gap, accompanied by a significant decrease in heat flows to the lower region of the ice shell. The results of numerical modeling of the process of melting an ice shell with a wire at different values of the convective heat transfer coefficient are presented. When analyzing the modeling results, conditions were identified for the occurrence of unacceptable overheating of a wire freed from ice. Various options for constructing a control system for the smelting process have been proposed to prevent overheating of the wire.

Numerical simulation study on heat transfer and fluid performance of novel calcium carbide heat collection system

This article proposes a novel recovery system for the substantial waste heat from the hot calcium carbide (CA). The study employs numerical simulation to investigate the heat transfer performance of the heat collection system (HCS). The research explores the effects of outlet position, inlet velocity, and inlet temperature on the operational characteristics and heat transfer performance of the HCS. The results indicate that optimal heat transfer performance is achieved when the outlet position is at the center of the collector. Instantaneous efficiency (IE), exergy loss (EL), pressure drop (PD), and heat transfer coefficient (HTC) increase with the velocity, while effectiveness decreases with the velocity. The peak values for IE, effectiveness, and HTC are 28.22%, 29.69%, and 1212.45 W/(m2*K), respectively. The heat transfer performance of the HCS is highest when the inlet temperature is 280K. The proposed CA HCS holds significant importance for recovering waste heat from electric stone production, providing a theoretical basis for subsequent experimental work.

Determination of the Effect of a Thermal Curtain Used in a Greenhouse on the Indoor Climate and Energy Savings

In order to reduce the impact of outdoor extreme weather events on crop production in winter, energy saving in greenhouses that are regularly heated is of great importance in reducing production costs and carbon footprints. For this purpose, the variations in indoor temperature, relative humidity and dew point temperature in the vertical direction (2 m, 4 m, 5.7 m) of thermal curtains in greenhouses were determined. In addition, depending on the fuel used, the curtains’ effects on heat energy consumption, heat transfer coefficient, carbon dioxide equivalents released to the atmosphere and fuel cost were investigated. To reach this goal, two greenhouses with the same structural features were designed with and without thermal curtains. As a result of the study, the indoor temperature and relative humidity values in the greenhouse with a thermal curtain increased by 1.3 °C and 10% compared to the greenhouse without a thermal curtain. Thermal curtains in the greenhouse significantly reduced fuel use (59.14–74.11 m3·night−1). Considering the heat energy consumption, the average heat energy consumption was 453.7 kWh·night−1 in the greenhouse with a curtain, while it was 568.6 kWh·night−1 in the greenhouse without a curtain. The average heat transfer coefficient (U) values were calculated at 2.87 W·m−2 °C with a thermal curtain and 3.63 W·m−2 °C without a thermal curtain greenhouse. In the greenhouse, closing the thermal curtain at night resulted in heat energy savings of about 21%, related to the decrease in U values. The use of a thermal curtain in the greenhouse reduced the amount of CO2 released to the atmosphere (116.6–146.1 kg·night−1) and fuel cost (USD 21.3–26.7·night−1). To conclude, extreme weather events in the outdoor environment adversely affect the plants grown in greenhouses where cultivation is performed out of season. A thermal curtain, used to reduce these adverse effects and the amount of energy consumed, is essential in improving indoor climate conditions, providing more economical greenhouse management and reducing the CO2 released into the atmosphere due to fuel use.

Flow condensation inside a multiport mini channel and a rectangular mini channel with pin fin array

In this paper, an experimental study of flow condensation was carried out of R134a in a multiport mini channel and a mini channel with pin fin array. The former comprised 20 parallel rectangular channels with an equivalent diameter of 0.64 mm. The latter is a narrow rectangular mini channel containing 10 rows of diamond pin fins with a staggered configuration, and height and longitudinal/transverse pitch of 0.5 mm and 2 mm respectively. To acquire the local value of heat flux and heat transfer coefficient, air jet impingement cooling devices were adopted to condense the vapor of refrigerants. The effects of vapor quality, mass flux, heat flux, and saturation pressure on flow condensation heat transfer coefficient were investigated with the operating conditions: vapor quality from 1 to 0, mass flux from 160 to 450 kg/(m2s), heat flux from 10.0 to 39.8 kW/m2, and saturation pressure from 600 to 1500 kPa. The experimental results indicated that the pin fin array significantly improves the condensation heat transfer coefficient up to nearly 186 % compared to the multiport mini channel in the high vapor quality region. For both channels, the local heat transfer coefficient increases with an increase in vapor quality, mass flux, and heat flux whereas decreases with increase in saturation pressure. The influence of heat flux and mass flux on heat transfer coefficient was more pronounced in the high vapor quality region than in the low vapor quality region. A sensitivity analysis was performed at different vapor quality levels. The most influential parameters in the smooth channel and the pin fin array are saturation pressure and mass flux, respectively. The relative contribution of heat flux is more obvious in the high vapor quality region. Some of the existing correlations have good prediction performance for the smooth channel experimental data in this paper, and the corresponding MAD is within 20 %. However, their deviations are much greater for the applications of pin fin array.

Experimental and numerical investigation of cooling effectiveness of nano minimum quantity lubrication

The literature on nano-minimum quantity lubrication (NMQL) machining suggests that its effectiveness is due to the increased thermal conductivity of the base oil and the reduced friction at the cutting interface due to the rolling effect of the nanoparticles. However, the relative contribution of the increased nanofluid thermal conductivity and nanofluid lubricity on the performance of NMQL has not been investigated. This study presents an experimental and a numerical investigation of NMQL cooling effectiveness and compares it with that of pure minimum quantity lubrication (MQL). A lumped system analysis was carried out on a flat Inconel workpiece with different heat inputs. Two different heat sources, an aluminum electric resistor and a propane torch, were used for three different cooling strategies: dry, MQL and NMQL. A propane torch was used to replicate the machining operational conditions involving vaporization of the base oil. The experimental results showed minimal improvements in the cooling effectiveness and the heat transfer coefficient of NMQL cooling when compared to MQL. There was less than a 1 % reduction in the surface temperature of the workpiece when using NMQL cooling compared to that of MQL. Additionally, the increase in the heat transfer coefficient for a flow rate of 22 ml/h was 1.07 %, while an increase of 1 % was observed at a flow rate of 44 ml/h. The experimental results suggest that the benefits of nano minimum quantity lubrication observed in the literature could be mostly associated with the lubrication benefits of the nanoparticles. This conclusion was derived from the experimental comparisons of the cooling effectiveness of NMQL over MQL, which yielded only minor improvements. Additionally, this paper presents a novel multiphase numerical model for NMQL. This model considered nanoparticles as distinct discrete particles along with the oil droplets, and it captured the flow conditions, average surface temperatures, and average heat transfer coefficient. This CFD model estimated the surface temperatures with errors below 10 %, while the heat transfer coefficient values had an average error of 12.05 %. This is the first attempt in the open literature to model a nano minimum quantity lubrication and cooling strategy using a two-phase approach.

Experimental investigation of heat transfer characteristics of inclined aluminium two phase closed thermosyphon

Abstract A reduction in the size of electronic equipment increases the heat generation rate. Failure of electronic equipment occurs if the heat is not dissipated properly. This paper examines the performance of aluminium two-phase closed thermosyphon for cooling electronic equipment. Acetone charged aluminium two-phase closed thermosyphon was fabricated with an inside diameter of 17.05 mm and 1 mm thickness. A series of experimentations were performed for inclination angles of 10°–90° at selected filling ratios of 30, 60 and 100 %, along with heat inputs of 100, 200 and 300 W. The condenser section flow rate of water was maintained constant. Minimum thermal resistance was obtained at a 30° inclination angle for all filling ratios and heat inputs. The evaporator and condenser sections have a maximum heat transfer coefficient at a 30° inclination angle. Thermosyphon, with a 30 % or 60 % filling ratio, performed better than a 100 % filling ratio for all inclination angles and heat inputs. As the heat input was increased, the heat transfer coefficients of the evaporator and condenser section were increased, whereas total thermal resistance decreased. For 300 W heat input and 30 % filling ratio, the minimum thermal resistance at a 30° inclination angle was 0.158 °C/W. It is found that, the same heat input and filling ratio, the maximum heat transfer coefficient value for the evaporator and condenser section at a 30° inclination angle was 1602 W/m2 °C and 5652 W/m2 °C, respectively.

Effect of Nozzle Parameters on the Cooling Conditions in the Secondary Cooling Zone of a Slab Caster

The following contribution focuses on the secondary cooling zone of a slab caster, analyzing the effects of nozzle parameters on cooling conditions. The research group employs experimental measurements, including the Nozzle Measuring Stand (NMS), to determine local heat transfer coefficients (HTC) and water distribution. These measurements are crucial for accurately setting the boundary conditions in the simulation. The implementation of these boundary conditions is realized by an Exponential Gaussian Process Regression (EGPR) to predict HTC values. The results highlight the importance of an accurate HTC assessment in regions with overlapping sprays. The developed methodology aids in enhancing the precision of simulations for continuous casting processes, contributing to a better product quality and process efficiency using the non-commercial software platform “m2CAST”.

Numerical simulation and optimization of heat pipes using nanoparticles: a feasibility study

This study investigated the effect of different particle sizes on the thermal effectiveness of a cylindrical screen mesh heat pipe using silver nanoparticles as the test substance. The study compared the effects of three different particle sizes (30, 50, and 80 nm) on heat transmission characteristics of the heat pipe. The results showed that the heat pipe’s thermal conductivity increases as the particle size decreases. The thermal conductivity measurements revealed 250 W/mK, 200 W/mK, and 150 W/mK for particles of 30, 50, and 80 nm sizes. Furthermore, the study analyzed thermal resistance of heat pipe for each particle size and found that as particle size decreases, thermal resistance decreases as well. The heat transfer coefficient values for particle sizes of 30, 50, and 80 nm were found to be 5500 W/m2K, 4500 W/m2K, and 3500 W/m2K, respectively. The ANOVA method was used to analyze the results statistically. The 30 nm particle size was found to have significantly higher thermal conductivity, lower thermal resistance, and a higher heat transfer coefficient than other particle sizes. Additionally, simulations were conducted to analyze the temperature variation of the hot source as a function of power dissipation for design and optimization.

Experimental thermal performance investigation of double pipe heat exchanger using MWCNT/water nanofluid

This paper presents an experimental study investigating the thermal performance of multiwall carbon nanotubes in a twin-pipe heat exchanger using a water-based fluid. The research followed a two-step method, where a concentration of 0.01 weight percent of SDS surfactant was utilized to prepare the carbon nanotubes. The heat transfer coefficient, friction coefficient, and overall heat transfer coefficient of the nanofluid were compared to those of the base fluid. The results revealed that the addition of a small amount of multiwall carbon nanotubes significantly enhanced the thermal conductivity and heat transfer coefficient of the water-based fluid. Moreover, the heat transfer coefficient exhibited an increase with higher Reynolds numbers. When the nanofluid flowed counter currently and was in the outer tube, the highest values for the overall heat transfer coefficient and the heat transfer coefficient were determined as 2864 and 7655.7 W/m2K, respectively. The findings indicate notable improvements in the thermophysical and thermal characteristics of the nanofluid.

Thermo-hydraulic rating of a shell and tube heat exchanger for acetone cooling using bell-delaware method

Shell and tube heat exchangers are becoming the most popular devices for the transfer of heat in industrial process applications. In the present work, the thermo-hydraulic rating of a shell and tube heat exchanger proposed to cool an acetone stream was carried out using the Bell-Delaware methodology. An overall heat transfer coefficient value of 426.70 W/m2.K, a calculated heat transfer area of 121.46 m2, and a percent excess area of 20.83% were obtained. Both the pressure drop of acetone and water had values of 1,622.36 Pa and 3,020.46 Pa, respectively. The proposed shell and tube heat exchanger can be used satisfactorily for the required application, since the percent excess area does not exceed the 25%, and the pressure drops of both fluid streams are below the limit values established by the process.

Experimental and Numerical Analysis of the Efficacy of a Real Downhole Heat Exchanger

In this paper, a three-dimensional (3D) numerical model based on the finite element method (FEM) is developed to determine the fluid flow and heat transfer phenomena in a real multi-tube downhole heat exchanger (DHE), designed ad hoc for the present application, considering natural convection inside a geothermal reservoir. The DHE has been effectively installed and tested on the island of Ischia, in southern Italy, and the measurements have been used to validate the model. In particular, the authors analyze experimentally and numerically the behavior of the DHE based on the outlet temperature of the working fluid, thermal power, overall heat transfer coefficient, and efficiency. Furthermore, the influence of the degree of salinity on the performance of the DHE has been studied, observing that it degrades with the increase in the degree of salinity. The results show that the DHE allows to exchange more than 40 kW with the ground, obtaining overall heat transfer coefficient values larger than 450 W/m2 K. At the degree of salinity of 180 ppt, a decrease in the efficiency of the DHE of more than 8% is observed.

Experimental study for artificial neural network modeling on thermal and flow performances of electric traction motor with oil spray cooling

The oil spray cooling is emerging as advanced thermal management technique for next generation electric traction motor. The performance of oil spray cooling needs to be mapped and predicted based on various influential factors to develop efficient thermal management system for electric traction motors. In the present work, the thermal and flow performances are experimentally investigated and predicted using an artificial neural network (ANN) for electric traction motor with oil spray cooling. The thermal and flow performances are extracted in terms of maximum temperature, heat transfer coefficient, Nusselt number, spray pressure and spray power consumption. The effects of nozzle types, spray heights, volume flow rates and chiller temperatures are analyzed as operating factors. The ANNs are modeled considering Levenberg-Marquardt (LM) training variant, 10 hidden neurons and Tangential-sigmoidal (Tan) and Logarithmic-sigmoidal (Log) as transfer functions. The thermal and flow performances are superior for full cone followed by hollow cone and spiral nozzles in decreasing order. The higher spray height, higher volume flow rate and lower chiller temperature show optimum values of maximum temperature and heat transfer coefficient. The spray power consumption is maximum in case of higher volume flow rate, reported less variation with change in spray height and chiller temperature. The ANN model comprising of LM-Tan algorithm is recommended as the best model with prediction error within ±1% for all thermal and flow performances. The predicted results from the optimum ANN model are validated with experiments with maximum coefficient of determination (R2) and coefficient of variance (COV) as 0.99 and 3.20, respectively. The correlations for thermal and flow performances in terms of all influential factors are proposed using the optimum ANN model.

Experimental Study on Indirect Liquid Cooling Performance of Metal 3D-Printed Cold Plates for Battery Thermal Management

Two new cold plates manufactured via metal 3D printing were experimentally investigated for thermal performance analysis in indirect liquid cooling operations; then they were compared to the traditional cold plates. Experiments were performed with different coolant inlet temperatures (15.7 °C and 24.5 °C) and ambient air velocities (0.5 m/s and 3 m/s) at tropical conditions; hereby, the impact of high dew point temperatures at tropics was also investigated. Body-centered cubic (BCC) and pillar elements were applied in the cooling cavity of the cold plates. The results showed that the target surface temperature in both BCC- and pillar-filled plate designs was maintained below the limits at the lower inlet temperature. However, at the higher inlet temperature, the temperature was only maintained below the limit when the ambient air velocity was 3 m/s. The convective heat transfer coefficient at the inlet temperature of 15.7 °C was found 1.5 and 2.5 times higher than the convective heat transfer coefficient value at the inlet temperature of 24.5 °C for the pillar- and BCC-filled plates, respectively. The performance evaluation criterion values were found in the range of 1.2 − 2.4, which depended on the operating conditions and were already higher than the referenced studies in the literature.

Experimental and numerical investigation of heat transfer and flow of water-based graphene oxide nanofluid in a double pipe heat exchanger using different artificial neural network models

In this study, a water-based graphene oxide nanofluid was utilized in a counter-current flow double pipe heat exchanger (DPHE). The inner pipe carried the hot fluid (deionized water-based fluid). In the outer pipe, the cold fluid was flown, in which the experiments were performed once for deionized water-based fluid and once for graphene oxide nanofluid. These experiments were conducted at various inlet hot fluid temperatures of 35, 45, and 55 °C, volumetric concentrations of 0.01, 0.055, and 0.1% for nanofluid, and cold fluid flow rates of 14.4, 18.9, and 23.4 ml/s to evaluate the thermohydraulics of the DPHE. The results demonstrated that an increase in the flow rate and concentration of nanoparticles led to an improvement in the heat transfer coefficient (HTC) . It is also observed that inlet hot fluid temperature had a negligible effect on this coefficient compared to the concentration and flow rate. Additionally, the results showed that the friction factor and pressure drop of nanofluid were higher than those of the basefluid and their values were increased by increasing the concentration. The maximum increase compared to the basefluid was 72% for the friction factor and 111%, for the pressure drop. Besides, radial basis function neural network (RBFNN) and multi-layer perceptron neural network (MLPNN) models were designed for estimating the HTC. Both models accurately predicted the coefficient, but the RBFNN model exhibited higher accuracy than the MLPNN model. The results further indicated that the nanofluid outperformed the basefluid in terms of HTC. Due to the exceptionally high thermal conductivity of graphene nanoparticles, the HTC of the nanofluid was improved by up to 85% compared to the basefluid. The optimum value of HTC was achieved at a temperature of 55 °C, a volume concentration of 0.1%, and a flow rate of 23.4 ml/s.

3D numerical investigation on flow boiling characteristics in large length-to-diameter ratio microchannel

To meet the demand of thermal management with large power and high heat flux in the future engineering applications, more and more attentions have been draw to develop the two-phase microchannel cooling technology. In this work, a three dimensional numerical investigation is performed to study the influence of mass flux and heat flux on the two phase flow boiling characteristics in a single microchannel with large length-to-diameter ratio. The related calculation results show that the Volume of Fluid (VOF) model coupled with Lee model can predict the two phase pressure drop and heat transfer coefficient of the microchannel well. And the flow patterns in the microchannel are classified as bubbly flow, slug flow and annular flow, which can be captured by this numerical model precisely. Besides, the transient pressure drop augments with the increase of mass flux. But the effect of mass flux on the amplitude of quasi-steady pressure drop and the peak value of time-averaged heat transfer coefficient along the microchannel is small. At last, the amplitude of quasi-steady pressure drop increases with the augmentation of heat flux. The maximum and fully developed time-averaged heat transfer coefficients along the microchannel increase with the heat flux.