Evaporator Walls Research Articles

Computational study of bubble, thin-film dynamics and heat transfer during flow boiling in non-circular microchannels

Flow boiling in multi-microchannel evaporators is one of the most efficient thermal management solutions for high-power-density applications. However, there is still a lack of understanding of the governing two-phase heat and mass transfer processes that occur in these devices, which has resulted in a limited availability of applicable boiling heat transfer prediction methods based on first principles, and of reliable thermal design tools. This article presents a systematic analysis of the dynamics of bubbles and the surrounding liquid film during flow boiling in three-side-heated non-circular microchannels. The study is performed using a custom version of ESI OpenFOAM v2106 with a geometric volume-of-fluid method to capture the interface dynamics, also incorporating conjugate heat transfer through the evaporator walls. The hydraulic diameter of the channel is fixed to Dh=0.229mm and the range of width-to-height aspect ratios ϵ=0.25−4 is examined. We investigate different fluids, namely water, HFE7100, R1233zd(E), R1234ze(E), and evaporator materials, namely copper, aluminium, silicon, stainless steel, with base heat fluxes in the range qb=50−200kW/m2. The results show that conjugate heat transfer acts to make the temperature distributions around the perimeter of the channel cross-section more uniform, and that the topography of the lubricating film and the extension of the dry vapour patches that develop while the film is depleted both depend on the cross-sectional channel shape and influence the heat transfer performance significantly. For highly wetting conditions, channels with ϵ=0.25 tend to allow enhanced heat transfer rates, with a spatially-averaged Nusselt number that is 50% higher than that obtained for ϵ=1 (square channels) and 10% higher than that for ϵ=4. This arises thanks to an extended evaporating film that covers the vertical walls which, owing to the three-side-heated configuration, contribute twice to the spatially-averaged heat transfer performance. For more hydrophobic conditions, large dry patches develop over the vertical walls for ϵ=0.25 due to the lower evaporator temperatures, leading to reduced heat transfer, with thermal performance weakly dependent on ϵ in the range ϵ=0.5−2.

Conjugate heat transfer effects on flow boiling in microchannels

• Heat distributes nonuniformly around the perimeter of microchannels in evaporators. • In single-phase flow, low aspect-ratio channels yield best heat transfer. • In two-phase flow, the Nusselt number trends vs. aspect-ratio are mixed. • High aspect-ratio channels exhibit larger thermal resistance and higher evaporator temperatures. • Conjugate heat transfer should be included in design models for evaporators. This article presents a computational study of saturated flow boiling in non-circular microchannels. The unit channel of a multi-microchannel evaporator, consisting of the fluidic channel and surrounding evaporator walls, is simulated and the conjugate heat transfer problem is solved. Simulations are performed using OpenFOAM v2106 and the built-in geometric Volume Of Fluid method, augmented with self-developed libraries to include liquid-vapour phase-change and improve the surface tension force calculation. A systematic study is conducted by employing water at atmospheric pressure, a channel hydraulic diameter of D h = 229 µm, a uniform base heat flux of q b = 100 kW / m 2 , and by varying the channel width-to-height aspect-ratio and channel fin thickness in the range ϵ = 0.25 –4 and W f = D h / 8 − D h , respectively. The effects of conjugate heat transfer and channel aspect-ratio on the bubble and evaporative film dynamics, heat transfer, and evaporator temperature are investigated in detail. This study reveals that, when the flow is single-phase, higher Nusselt numbers and lower evaporator base temperatures are achieved for smaller channel aspect-ratios, from Nu ≃ 4 and T b − T s a t ≃ 9 K when ϵ = 4 , to Nu ≃ 6 and T b − T s a t ≃ 2 K when ϵ = 0.25 , for same fin thickness W f = D h / 8 . In the two-phase flow regime, Nusselt numbers in the range Nu = 12 − 36 are achieved. The trends of the Nusselt number versus the aspect-ratio are non-monotonic and exhibit a marked dependence on the channel fin thickness. For small fin thicknesses, W f = D h / 8 and W f = D h / 4 , an overall ascending trend of Nu for increasing aspect-ratios is apparent, although in the narrower range ϵ = 0.5 –2 the Nusselt number appears weakly dependent on ϵ . For thicker fins, W f = D h / 2 and W f = D h , the Nusselt number decreases slightly when increasing the aspect-ratio in the range ϵ = 0.5 –2, although this trend is not monotonic when considering the entire range of aspect-ratios investigated. Nonetheless, due to conjugate heat transfer, Nusselt numbers and evaporator base temperatures follow different trends when varying the aspect-ratio, and channels with ϵ < 1 seem to promote lower evaporator temperatures than higher aspect-ratio conduits.

Effect of evaporator numbers on water production of a free convection air-water harvester

An experimental study was performed to determine the effect of the number of evaporators on the water production and coefficient of performance of an air-water harvester. Water was obtained on the evaporator walls in free convection and condensation heat transfer modes. The evaporator dimension was 480 mm × 285 mm × 6.25 mm, and it was made of copper pipes. The evaporators were installed in a parallel arrangement in an open box (top and bottom open) with a size of 500 mm × 500 mm × 500 mm. The refrigerant R134a was used as the working fluid. The experiments were performed at a low pressure of 40 psi and a high pressure of 180 psi. In the outside evaporator walls, air flowed naturally and part of the water vapour in the air condensed. Three evaporator variations were tested: Case A (one evaporator), Case B (two evaporators), and Case C (three evaporators). Each evaporator contained 25 copper pipes and the length of each pipe was 285 mm. The maximum water production and the evaporator efficiency attained were 0.51 L a day and 13 % respectively. The COP of the machine was ranging from 5.2 to 13.3. Increasing the evaporator number elevated the water production.

Loop thermosyphons with porous coatings

The aim of this work is to experimentally evaluate the heat exchange of evaporation and condensation inside a new annular thermosyphon with horizontally located zones of evaporation and condensation, which are interconnected by a vapor line and a condensate line. The mini-tube of the liquid line is introduced coaxially into the cylindrical evaporator with a porous coating, thus an annular channel is formed between the tube and the porous coating of the evaporator. The purpose of this study is to determine the thermal resistance of the device and its components, as well as the temperature field along the thermosyphon. The considered thermosyphon guarantees a shorter start-up time, lowers the temperature of the evaporator walls, has a small temperature hysteresis with an increase/decrease in the transferred heat flux, and suppresses temperature pulsations inside an evaporator. The 130 mm long evaporator is made of copper tube (d out = 12 mm, d in = 10 mm). The wick is obtained by sintering copper powder. The working fluids R245fa and R600 were used. The thermal resistance of thermosyphon does not exceed 0.5 K/W, and of the evaporator 0.1–0.05 K/W).

Determination of High Temperature Corrosion Rates of Steam Boiler Evaporators Using Continuous Measurements of Flue Gas Composition and Neural Networks

The use of low-emission combustion techniques in pulverized coal-fired (PC) boilers are usually associated with the formation of a reduced-gas atmosphere near evaporator walls. This increases the risk of high temperature (low oxygen) corrosion processes in coal-fired boilers. The identification of the dynamics and the locations of these processes, and minimizing negative consequences are essential for power plant operation. This paper presents the diagnostic system for determining corrosion risks, based on continuous measurements of flue gas composition in the boundary layer of the combustion chamber, and artificial intelligence techniques. Experience from the implementation of these measurements on the OP-230 hard coal-fired boiler, to identify the corrosion hazard of one of the evaporator walls, has been thoroughly described. The results obtained indicate that the continuous controlling of the concentrations of O2 and CO near the water wall, in combination with the use of neural networks, allows for the forecasting of the corrosion rate of the evaporator. The correlation between flue gas composition and corrosion rate has been demonstrated. At the same time, the analysis of the possibilities of significantly simplifying the measurement system by using neural networks was carried out.

Steady-state analysis of a Capillary Pumped Loop for Terrestrial Application with methanol and ethanol as working fluids

This paper aims to study and understand the behaviour of a CPLTA (Capillary Pumped Loop for Terrestrial Applications) in different operating conditions. This CPL, designed for power electronics cooling applications, has been analysed from two different points of view by comparing its operating behaviour with Ethanol and Methanol as working fluids. The influence of the fluids on CPLTA was experimentally explored by modifying: the thermal characteristics (applied heat flux density on the evaporator walls, saturation temperature in the reservoir and condenser inlet temperature) and the hydraulic characteristics (by acting on a setting valve to generate controlled and increasing pressure losses in the vapour line) in order to show how they impact the behaviour of the loop. Thermal aspects of the loop were analysed, including changes of evaporator conductance. To obtain the thermodynamic cycles and evaporator conductance, based on the real evaporation temperature in the porous wick, a theoretical study was conducted using a specific post-processing tool.

Design of a thermosyphon-based thermal valve for controlled high-temperature heat extraction

Conventional concentrated solar power (CSP) is a reliable alternative energy source that uses the sun’s heat to drive a heat engine to produce electrical power. An advantage of CSP is its ability to store thermal energy for use during off-sun hours which is typically done by storing sensible heat in molten salts. Alternatively, thermal energy may be stored as latent heat in a phase-change material (PCM), which stores large quantities of thermal energy in an isothermal process. On-sun, the PCM melts, storing energy. Off-sun, the latent heat is extracted to produce dispatchable electrical power.This paper presents the design of a thermosyphon-based device with sodium working fluid that is able to extract heat from a source as demand requires. A prototype has been designed to transfer 37kW of thermal energy from a 600°C molten PCM tank to an array of 9% efficient thermoelectric generators (TEGs) to produce 3kW of usable electrical energy for 5h. This “thermal valve” design incorporates a funnel to collect condensate and a central shut-off valve to control condensate gravity return to the evaporator. Three circumferential tubes allow vapour transport up to the condenser.Pressure and a thermal resistance models were developed to predict the performance of the thermal valve. The pressure model predicts that the thermal valve will function as designed. The thermal resistance model predicts a 5500× difference in total thermal resistance between “on” and “off” states. The evaporator and condenser walls comprise 96% of the “on” thermal resistance, while the small parasitic heat transfer in the “off” state is primarily (77%) due to radiation losses.This simple and effective technology can have a strong impact on the feasibility, scalability, and dispatchability of CSP latent storage. In addition, other industrial and commercial applications can benefit from this thermal valve concept.

Clarification of Bitter Orange (Citrus Aurantium) Juice Using Microfiltration with Mixed Cellulose Esters Membrane

Microfiltration of bitter orange juice was performed with three levels (0.3, 0.6 and 0.9 bar) of transmembrane pressures (TMP) and three feed velocities (0.3, 0.9 and 1.2 m/s) to evaluate their effects on the resulting permeate flux, membrane fouling (using Hermia's model) and physicochemical properties of the juice during clarification. Results showed that TMP and feed velocity had important effects on the permeate flux and creation time of each fouling mechanism. The permeate flux increased while increasing of the TMP and feed velocity. Although membrane clarification decreased turbidity, acidity, ascorbic acid and total soluble solid content of bitter orange juice; its polyphenol components and antioxidant activity did not change significantly. Microfiltration eliminates more than 90% of the particles in bitter orange juice and preserves its main nutritional compounds successfully. Practical Applications Large particles in extracted bitter orange juice will be burnt on evaporator's walls during juice concentration and generate off flavor in the final product. While microfiltration of fresh juice is a suitable method to eliminate these particles and prevent unwanted off flavors during juice evaporation, fouling phenomena decreases process performance. Increasing the feed velocity in all stages of process and the transmembrane pressure in first stages of clarification treatment are two useful tools to increase process efficiency.

Conjugate heat transfer analysis using a simplified household refrigerator model

Air circulation and heat transfer in a simplified household refrigerator model was numerically studied. Three refrigerator configurations were analyzed and compared to highlight the influence of inner structures. The results show that radiation enhances heat transfer between evaporator and inner walls, resulting in a higher temperature gradient across the insulation foams and a higher heat loss rate. Radiation also weakens the natural convection but increases the air circulation between shelves. A vertical temperature stratification is observed in the fresh food compartment for all configurations. The shelves and food strengthen air circulations between the shelves and inner walls, which increases vertical temperature gradient and decreases the temperature homogeneity in the compartment.

Modeling of Gray Gas Radiation With Different Shapes of Heat Source in Three-Dimensional Enclosures

This article deals with performance evaluation of the discrete ordinates method in terms of its capacity to provide accurate results in solving radiation mode problems with different complexities. The problem formulation is done keeping in view the heat transfer process in an evaporator used in a coal-based thermal power station. The hot gas mass in the evaporator is assumed to be a heat source with two different shapes, spherical and conical. The evaporator walls that are covered with a water cooling jacket are modeled with a convective boundary condition. The gas is assumed to be gray and absorbing–emitting. The solution of the radiative transport equation is carried out using the discrete ordinates method. Parametric studies are performed for a wide range of aspect ratio, extinction coefficient, and convective heat transfer coefficient. The code is validated by comparing the result of discrete ordinates method with the exact and the discrete transfer method for nonradiative as well as radiative equilibrium conditions.

Transient analysis of boiling heat transfer in periodic drying out miniature pools

The evolution of the two-phase thermal control technique is moving in the direction of the use of devices which operate in stable periodic or chaotic unsteady regimes. In these devices the heat transfer coefficient relative to the evaporator walls therefore changes over time and it is hardly predictable, especially in the case of boiling regime. This paper deals with the analysis of the boiling coefficient evolutions over time during the operations of a thermal control devices which periodically operates, the Periodic Two-Phase Thermosyphon (PTPT). An experimental technique for measuring the transient boiling heat transfer coefficient in a thick flat evaporator is shown. This technique has been used to measure boiling heat transfer coefficients over time in the case of evaporators with small pools of liquid (lower than 64 ml) which periodically dries out. The FC72 fluid has been used in the experiments. The results show that no nucleate boiling regime has been observed in the evaporator, which works in an unstable transitional boiling regime with relative heat transfer coefficients lower than those typical of nucleate boiling regimes (lower than 50%).

Systematic Investigation of Deposits in the Boiler of a Semitechnical Rotary Kiln Incinerator—Mineralogical and Chemical Characterization

Mineralogical and chemical characterizations of deposits from the evaporator walls in the boiler of the pilot plant THERESA indicate a correlation of increased corrosion processes with phase transformation in the mineralogical composition. Samples were taken in horizontal and vertical profiles which allow a correlation of the variations in flue gas properties (e.g., temperature, chemical composition) as well as a comparison of the composition in the same boiler zone. The coatings were composed mainly of different sulfates. Phase transformation between calcium- (Ca), sodium- (Na), potassium- (K), and lead- (Pb) sulfates can be observed together with macroscopic corrosion damages at the evaporator walls. These findings should help to give a prognosis of the most endangered areas in the boiler and therefore minimize expenditures for boiler maintenance.

Analysis of the heat transfer coefficient of grooved heat pipe evaporator walls

A model for the radial heat transfer of a grooved heat pipe evaporator is presented. It combines the solution of a two-dimensional heat conduction problem with the calculation of the shape of the liquid-vapour interface and its temperature, taking into account the influence of meniscus curvature and adhesion forces on the volatility of the liquid. It is shown that the common assumption of an interface temperature equal to the saturation temperature of the vapour can lead to a large overprediction of the radial heat transfer coefficient.