Flow Boiling Characteristics Research Articles

Effect of RIBS/FINS and Aspect Ratio on Flow Boiling Characteristics in Conventional Channels

Abstract In this work, experiments are conducted with conventional rectangular channels of two different aspect ratios (AR = w/d) for the horizontal boiling flow conditions at atmospheric pressure. Distilled water was used as the working substance. The heat transfer coefficients (HTC) were measured for mass fluxes and heat fluxes ranging from 85.94 kg/m2-s to 343.77 kg/m2-s and 10 kW/m2 to 100 kW/m2, respectively, and at inlet subcooled temperatures of 303 K, 313 K, and 323 K. Visualization of the boiling phenomenon was done using a high-speed camera for the two channels under similar conditions. The results show that the AR has a dominant effect on the HTC. At low heat flux values, higher HTC was noticed for the channel of higher AR (AR = 1.25) whereas, at high heat flux conditions, the HTC is higher for the channel of lower AR (AR = 0.2). With an increase in inlet subcooled temperature, the HTC decreased for both channels due to increased thermal boundary layer thickness and reduced bubble formation. Further, the channel of AR = 1.25 with ribs/fins performed better than the smooth channel due to the high bubble nucleation rate.

Comparison of Flow Boiling Characteristics between Minichannel and Macrochannel in Solar Still-Numerical Study

The thermal performance of a rectangular minichannel has been estimated for various heat fluxes (1 kW/m2 to 2 kW/m2) and fluid mass flux rates (20 g/s to 40 g/s). The mixture model is used to determine two-phase flow characteristics in the mini-macro channel. The focus of the present work is to establish prime factors for the enhancement of vapor fractions inside the minichannel (<3 mm) and macrochannel (>3 mm). The fluid flow characteristics are explained with help of temperature difference (the wall side and water film side), friction factor, Nusselt number, evaporative thin film thickness, heat transfer coefficient and vapor fraction. Validation of current simulation with earlier research article shows good agreement. The friction factor value for the minichannel and macrochannel are 0.0004 and 0.0011, respectively. It is found that heat transfer is significantly more in macrochannel pipes compared with minichannels. By using the dimensional analysis, empirical correlation is developed for vapor fraction with dependent parameters such as channel diameter, fluid mass flow rate and heat flux the first time. The predicted correlation of vapor fraction and film thickness help to understand the pronounced effect of channel diameter on heat transfer.

Validation of the Eulerian–Eulerian Two-Fluid Method and the RPI Wall Partitioning Model Predictions in OpenFOAM with Respect to the Flow Boiling Characteristics within Conventional Tubes and Micro-Channels

Flow boiling within conventional, mini and micro-scale channels is encountered in a wide range of engineering applications such as nuclear reactors, steam engines and cooling of electronic devices. Due to the high complexity and importance of the boiling process, several numerical and experimental investigations have been conducted for the better understanding of the underpinned physics and heat transfer characteristics. One of the most widely used numerical approaches that can analyse such phenomena is the Eulerian–Eulerian two-fluid method in conjunction with the RPI model. However, according to the current state-of-the-art methods this modelling approach heavily relies on empirical closure relationships derived for conventional channels, limiting its applicability to mini- and micro-scale channels. The present paper aims to give further insights into the applicability of this modelling approach for non-conventional channels. For this purpose, a numerical investigation utilising the Eulerian–Eulerian two-fluid model and the RPI wall heat flux partitioning model in OpenFOAM 8.0 is conducted. Initially the parameters comprising the empirical closure relationships used in the RPI sub-models are tuned against the DEBORA experiments on conventional channels, through an extensive sensitivity analysis. In the second part of the investigation, numerical simulations against flow boiling experiments within micro-channels are performed, utilising the previously optimised and validated model setup. Furthermore the importance of including a bubble coalescence and break-up sub-model to capture parameters such as the radial velocity profiles, is also illustrated. However, when the optimal model setup, in conventional tubes, is used against micro-channel experiments, the need to develop new correlations from data obtained from mini and micro-scale channel studies, not from experimental data on conventional channels, is revealed.

Numerical Analysis of Flow Boiling Characteristics of a Single Channel Heat Sink Subjected to Multiple Heat Sources

High-power electronic devices with multiple heat sources often require temperature uniformity and to operate within their functional temperature range for optimal performance. Micro-channel cooling could satisfy the heat dissipation requirements, but it may cause temperature non-uniformity. In this paper, simulations are performed for different geometric parameters of the channel and the position of the heat source. The results show that a flattened channel can effectively reduce the heat source temperature, broadening the straight channel can reduce the flow resistance and enhance heat transfer, while widening the channel at the bend may lead to local dryness. Meanwhile, a thermal model is established to analyze the influence of the position of the heat source. The results also show that with the increase in the curved channel radius, the phenomenon of vapor–liquid separation becomes more obvious, the pressure drop decreases, but the heat transfer effect worsens.

Experimental Study on Flow Boiling Characteristics in Continuous and Segmented Microchannels with Vapor Venting Membrane

Flow boiling in microchannels is one of the promising techniques for heat dissipation occurred in micro devices. However, the rapid bubble growth must be suppressed, which leads to serious boiling instabilities, high pressure drop, and low heat transfer coefficient. The addition of porous hydrophobic membrane has proven an effective method to remove the vapor in-site in the literature. However, the effects of heat sink’s topological structures on the vapor venting are still a research gap. The present study experimentally investigates the influence of Polytetrafluoroethylene (PTFE) membrane on fluid flow pattern, pressure drop, vapor venting performance, and heat transfer characteristics of flow boiling in the continuous and segmented heat sinks. Results show that the vapor venting membrane can reduce the pressure drop and increase the heat transfer coefficient effectively by decreasing the exit vapor quality, especially in combination with the segmented structures. The interconnection area as a space for bubble growth and coalescence is beneficial for vapor venting due to increased vapor pressure and quantity. Following the enhanced vapor discharge, the fluctuation of pressure drop is further weakened, which is conducive for the safe operation of heat sink.

Experimental Investigation of Flow Boiling Characteristics in Counter-Flow Microchannels with Different Mass Flux Distributions

Experimental Investigation of Flow Boiling Characteristics in Counter-Flow Microchannels with Different Mass Flux Distributions

Study on Flow Boiling Characteristics in Rectangle Channel After Formation of Blisters

Plate-type fuel elements is one of the first fuel structure choice for the novel integrated PWR, however, blisters will appear on the cladding induced by irradiation and fission. In this work CFD method was used to investigate the subcooled boiling characteristic of the water in rectangle channel with round and pillow blisters, the modified RPI model was also proposed, we can draw conclusions as follows: In the channel with round blister, as the blisters will increase the local flow resistance and more fluid will flow through center of the channel. Boiling occurred only in the area near the edges, nearly no vapor appeared at the center of the channel. The boiling region in channel with pillow shape blisters is wider and concentrated between two pillow blisters and downstream of the non-blisters side. The dry out area are both in the downstream region of blisters for the two types of channels.

Experimental Investigation on the Change in Flow Boiling Characteristics Due to Boiling-Induced Copper Ageing

Abstract Heat sinks and heat exchangers based on flow boiling in mini/microchannels are expected to be more compact and efficient. One of the major challenges while using copper material for phase-change heat transfer application is the change in surface characteristics after prolonged usage due to the thermal oxidation of surface over time. This study involves the repeated experimental runs of flow boiling of water in a 1 mm hydraulic diameter end-milled copper channel to verify the influence of ageing on the thermal and hydraulic performance. As it is difficult to measure the surface wettability in a mini/microchannel, this work makes use of the ageing and surface characterization study conducted on the dummy copper samples to infer the influence of ageing on mini/microchannel surface characteristics and consequently its boiling performance. The test involves measuring over a period of time the wetting behaviors of the end-milled copper samples left in water at three different conditions: one in a constant temperature bath maintained at 60 °C and 1 atm and the remaining two in a pool of boiling water at 110 °C and 120 °C. The study compares the fresh sample and the aged sample for the surface oxidation, surface chemical composition and surface morphology, and discusses the changes in the contact angle and surface morphology caused by copper ageing.

Flow Boiling Characteristics of Nanofluids in Microchannels

Mikrocihazlarda giderek artan isi akisi degerleri nedeniyle geleneksel isi transfer arttirim yontemleri mikrosistemlerin sogutulmasinda yetersiz kalmaktadir. Kaynama ile isi transferi yuksek isi transfer katsayilarinin elde edilmesi yaninda yuzey sicakliginin neredeyse sabit olmasi nedeniyle yuksek isi atimini gerektiren mikrosistem uygulamalari icin onemli bir potansiyele sahiptir. Diger yandan geleneksel akiskanlar yerine nanoakiskanlarin kullanimi yuksek isil performans icin onemli bir alternatif olarak dusunulmektedir. Nanoakiskanlarin tek fazli isi transferiyle ilgili bircok calisma olmasina ragmen mikrokanallarda kaynamali akis durumunda nanoakiskan kullanimiyla ilgili sinirli sayida calisma mevcuttur. Literaturde nanoakiskanlarin kaynamali akista kullaniminin bazi calismalara gore uygun olup olmadigi yonunde belirsizlikler vardir. Bu nedenle nanoakiskanlarin kaynamali akis sartlarinda kullanilabilirligi ile ilgili daha kapsamli calismalara ihtiyac vardir. Bu calismada, farkli nanoakiskanlar ile yapilan deneysel calismalar derlenmis ve nanoakiskanlarin mikrokanallarda kaynamali akista kullaniminin avantaj ve dezavantajlari belirlenmeye calisilmistir.

Experimental investigation of flow-boiling characteristics in a single micro-channel with inlet cavitation structure

This research article investigates the effect that hydrodynamic cavitation has on flow-boiling characteristics in a single micro-channel. Flow and heat transfer characteristics have been investigated over a range of effective heat fluxes (71–330 W/cm2) and vapor quality (0.25–0.92) over a broad range of mass flow rates. The images of flow condition have been photographed. A fully developed cavitation flow, including inception, growth, and collapse of bubble, can be presented in the micro-channel. The intensity of bubble and length of the two-phase region increase with increase in the mass flow rates, and bubble flow pattern dominates the fully developed cavitation flow. Heat transfer coefficient has also been calculated at experimental conditions, and the coefficients decrease with increase in the vapor quality, but increase with increase in the heat fluxes.

Research on the Flow Boiling Characteristics of Water in a Multi-Furcated Tree-Shaped Mini-Channel

A new multi-furcated structure of tree-shaped mini-channel was proposed in this paper. Numerical simulation was conducted to analyze the flow boiling and bubble growth characteristics of water in the mini-channel. By using the constant wall temperature heating method, the model was divided into different sub-regions according to different wall temperatures. Typical bubbly flow and slug flow patterns were obtained through the monitoring of the boiling process. The simulation results showed that when the convective heat transfer dominated, the pressure drop maintained minor and stable; while the nucleate boiling dominated, the pressure drop increased significantly and fluctuated within a certain range. Moreover, with the increase of vapor quality, the heat transfer coefficient increased first and then decreased. It is also found that the geometric structure of the multi-furcated tree-shaped mini-channel had a great impact on the heat transfer characteristics, and simulation results agreed well with the theoretical values.

Flow Boiling Characteristics for R1234ze(E) in 1.0 and 2.2 mm Circular Channels

Experimental flow boiling heat transfer results are presented for horizontal 1.0 and 2.2 mm I.D. (internal diameter) stainless steel tubes for tests with R1234ze(E), a new refrigerant developed as a substitute for R134a with a much lower global warming potential (GWP). The experiments were performed for these two tube diameters in order to investigate a possible transition between macro and microscale flow boiling behavior. The experimental campaign includes mass velocities ranging from 50 to 1500 kg/m2 s, heat fluxes from 10 to 300 kW/m2, exit saturation temperatures of 25, 31 and 35 °C, vapor qualities from 0.05 to 0.99 and heated lengths of 180 mm and 361 mm. Flow pattern characterization was performed using high speed videos. Heat transfer coefficient, critical heat flux and flow pattern data were obtained. R1234ze(E) demonstrated similar thermal performance to R134a data when running at similar conditions.

An Experimental Investigation of Flow Boiling Characteristics of Water in Parallel Microchannels

AbstractMicrochannels are being considered in many advanced heat transfer applications including automotive and stationary fuel cells as well as electronics cooling. However, there are a number of fundamental issues from the heat transfer and fluid mechanics perspectives that still remain unresolved. The present work focuses on obtaining the fundamental heat transfer data and two-phase flow patterns present during flow boiling in microchannels. An experimental investigation is performed for flow boiling using water in six parallel, horizontal microchannels with a hydraulic diameter of 207 μm. The ranges of parameters are: mass flux from 157 to 1782 kg/m2s, heat flux from 5 to 930 kW/m2, inlet temperature of 22°C, quality from sub-cooled to 1.0, and atmospheric pressure at the exit. The corresponding single-phase, all-liquid flow Reynolds number range at the saturation conditions is from 116 to 1318. The measured single-phase, adiabatic pressure drop agreed with the conventional theory within the experimental error. The experimental single-phase Nusselt number was found to be between the constant heat flux and the constant wall temperature boundary conditions, corresponding to NuH and NuT respectively. The flow visualization demonstrates that the flow reversal condition in parallel flow channels is due to bubble nucleation followed by its rapid growth. In addition, the dry-out condition is observed, showing a change in the contact angles of the liquid-vapor interface. The local flow boiling heat transfer coefficient exhibits a decreasing trend with increasing quality. A comparison with the nucleate boiling dominant regime of a flow boiling correlation shows good agreement, except for the large peak in two-phase heat transfer coefficient observed at the onset of nucleate boiling.