Hot Tube Research Articles

Thermal dynamic and failure research on an air-fuel heat exchanger for aero-engine cooling

This paper presents a novel air-fuel heat exchanger used for the cooled cooling air technology in aero-engines. The helical tube heat exchanger weighing 1.1 kg with an area density of 214 m2/m3 can cool the hot air down 260 K at the air flow of 0.3 kg/s with relative airside pressure drop less than 0.6%. Empirical correlations by multiplying a constant of 1.06 and 0.837 can well predict the pressure drop and convective heat transfer coefficient respectively for hot gas cross-flowing helical tube bundles. Furthermore, in the tube failure tests to simulate the fuel control system fault and flight mode conversion, the straight tube can work continuously for more than 360 s under the extremely high tube wall temperature of over 1200 °C in the heat flux sudden increase test, while the bent tube could continue to glow brightly for 30 min. Coke morphology and chemical composition analysis revealed that increased thermal stress caused by gradually thicker coke layer takes a major cause in the heat flux sudden increase test. The secondary flow in bent tubes can effectively improve the convective heat transfer performance and reduce the formation of coke deposition, thereby improving the tube working life.

REWETTING AND TRANSIENT HEAT TRANSFER ON THE HEATED HORIZONTAL TUBE SURFACE DURING THE AIR-ATOMIZED SPRAY COOLING

The purpose of this study is to investigate the cooling and rewetting of a heated horizontal tube surface with an air-atomized water spray impingement. Rewetting and transient heat transfer are crucial in nuclear reactor safety during a postulated accident, such as cooling of hot calandria tubes (CT) during the large-break loss of coolant accident (LOCA). The rewetting velocity in the circumferential direction and the rate of cooling of the heated tube surface were determined as a function of nozzle operating parameters. To estimate the local spray impingement density on the tube surface, an in-house mechanical patternator was designed and developed. To record the flow state during cooling, a high-speed video camera was used. The rewetting velocity on the tube surface was determined using the outcome of thermocouples mounted on the heated tube wall and an imaging system used to record the video picture during the runs. The two techniques of calculating rewetting velocity are compared. The highest heat flux removed from the tube surface was estimated as 3.7 MW/m<sup>2</sup>, and the maximum rewetting velocity was found to be 5.58 mm/s. An excellent agreement regarding rewetting velocity has been reported utilizing thermocouples and a high-speed camera.

Thermoelectric Material Fabrication using Mask Image Projection Based Stereolithography Integrated with Hot Pressing

Abstract: Waste energy harvest using thermoelectric (TE) materials will be a potential solution to the serious environmental pollution and energy shortage problems. Due to limitations of current manufacturing techniques in geometry complexity and high density, TE devices are not widely utilized in daily life to gather waste energy. 3D printing brings an opportunity to solve the fabrication limitations. In this paper, a hybrid process was developed to fabricate thermoelectric materials by integrating hot pressing with stereolithography. The mold and punch were designed and printed to fabricate thermoelectric devices used on hot water tubes via stereolithography. The Sb2Te3 powders filled the 3D printed mold in a layered manner, and each layer of powders was compacted under the pressing of punch at a certain temperature and compressive force. The polymer mold was removed after the sintering process to form the final TE components. A series of experiments were conducted to identify the optimal heating temperature and compressive force. The microstructures morphology and electrical conductivity of fabricated Sb2Te3 samples were evaluated. This research work conducted a scientific investigation into the fabrication of TE material with a hybrid process, including hot pressing and 3D printing, to solve the current manufacturing challenges, providing perspectives on developments of TE devices used in various energy harvest applications.

Conjugated non-Newtonian phase change process in a shell and tube heat exchanger: A parametric-geometric analysis

The present study aims to better knowledge of the phase change process in shell and tube heat exchangers in the presence of thick tubes, porous medium, and Carreau- Yasuda non-Newtonian fluid. It is the first time that this combination has been presented in the literature. It is believed that considering the phase change process of RT-35 as a Carreau-Yasuda non-Newtonian one in the presence of porous medium and copper pipes can create a challenge in the field. The continuity and momentum equations along with the horizontal and vertical directions, the heat equation for RT-35 as a non-Newtonian Carreau–Yasuda fluid, and the heat equation for thick tubes were determined and transformed into their non-dimensional forms, which were then solved using the finite element method. The effects of non-dimensional numbers such as Stefan number, porosity parameter, Rayleigh number, and Effect of rheological behavior of fluid have been determined. Reaching better performance via using porous medium and center- a to- center distance of double pipes, as well as a number of the pipes, are the ideas to conduct the present study. The results were reported as the average Nusselt number, melting fraction of the PCM, the average velocity of the non-Newtonian fluid, streamlines, and isotherms, all as functions of non-dimensional time. The results indicated that higher values for the porosity parameter led to lower heat transfer rates, which in turn weakened the PCM melting front. It was also found that changing the center-to-center distance between the hot tubes influenced melting front propagation.

Energy-saving production of high value-added foamed glass ceramic from blast furnace slag and hazardous wastes containing heavy metal ions

In this work, foamed glass ceramic (FGC) is prepared by using blast furnace slag (BFS), waste glass, and zeolite adsorbing heavy metal ions (HMI) as the raw materials. To conserve energy, the residual heat of BFS at low-temperature could be used as a heat source. In order to simulate the heating process, the green body is directly put into a hot tube furnace and heated during the cooling process without heat preservation process. The optimized FGC has an apparent density of 0.81 g cm−3, a compressive strength of 4.51 MPa, and a water absorption of 19.5%. The physical property of high value-added FGC meets the industry standard and it could be widely used in the field of building materials. The leaching efficiencies of Cu2+, Zn2+, and Cd2+ of FGC are as low as 0.76 mg L−1, 0.55 mg L−1, and 0.79 mg L−1, respectively. The HMIs are steadily stabilized in the FGC and have less impact on the environment. It is found that the newly formed columnar crystal has a great contribution to the improvement of compressive strength due to its pinning effect between the glass phase and crystalline phase. The newly formed lamellar crystal plays an important role in preventing HMI from escaping out into the environment. This study provides a reasonable method for converting solid waste into high value-added product, recycling industrial tail heat at low-temperature zone, and eliminating the harm of hazardous waste containing HMI to the environment.

Gas-jet HFCVD synthesis of diamonds from mixtures of hydrogen with ethylene and methane

The gas-jet deposition method (a modification of the Hot Filament Chemical Vapor Deposition method (HFCVD)) was used to deposit some diamond structures on a molybdenum substrate from a hydrogen-ethylene mixture activated on hot tungsten. The experiments were performed for two lengths of activation reactors at different ethylene flow rates. For comparison, some diamond structures were synthesized from a hydrogen-methane mixture under similar conditions. The resulting structures were studied with the scanning electron microscopy and Raman spectroscopy methods. The reacting mixture flow through the heated catalytic channel was simulated by solving the Navier-Stokes equations accounting for homogeneous and heterogeneous chemical reactions. The reactor (hot tungsten tube) length and diameter effects as well as carbonaceous admixture supply effects were analyzed. The main diamond precursor's formation pathways were analyzed and the reaction chains' complexity was demonstrated. The simulation results are in good agreement with the experimental data. The numerical experiments made it possible to reveal the growth process key driving parameters.

Isothermal hot tube material characterization – Forming limits and flow curves of stainless steel tubes at elevated temperatures

A new isothermal hot-tube-bulge test as well as a modified pressurized-tube-tensile test are presented to characterize forming limits of tubular materials at elevated temperatures – up to 1000 °C. By controlling temperature, internal pressure, and axial force the complete forming limit curve is determined under isothermal, frictionless conditions and nearly linear strain paths. In the hot-tube-bulge test, a tangential localization forms before failure, marking a forming limit. In the pressurized-tube-tensile test, an axial localization occurs. For the former, a new method for estimating the forming limit was developed whereas the later can be evaluated by standardized methods. The resulting forming limits are at comparable levels to those known from literature, while the new hot-tube-bulge test overcomes the uncertainty which arises from friction in other test setups. The materials used for the investigations are the ferritic and martensitic stainless steels X2CrTiNb18 and X12Cr13, respectively. The hot-tube-bulge test is examined for its suitability in determining flow curves. An empirical-analytical model is introduced to determine the stress components. It is based on tangential and meridional force equilibria, taking the internal pressure and the projected effective areas into account. Flow curves up to effective plastic strains of 0.5 are determined, exceeding the maximum strain of common hot tensile-test by a factor of 2. The strain-rate dependent stress-strain data was used to identify the parameters of a simplified Hensel-Spittel approach. This allows the prediction of isothermal flow curves with varying strains and strain rates at a temperature of 1000 °C.

Ignition of an ionic liquid dual-mode monopropellant using a microwave plasma torch

The blend of hydroxylammonium nitrate (HAN) and imidazole-based ionic liquid has shown good feasibility in chemical and electric dual-mode green space propulsion. As conventional catalyst-driven ignition faces challenges arising from long preheat delay and catalyst failure, this work designs a novel ignition actuator using a microwave plasma torch. The precise tuning of the ¼-λ resonant cavity and improvement in the quality factor (QF) of the microwave igniter make the device efficient and thus allow the propellant to be ignited at a power of approximately 100 W. In the present reduced test rig, the maximum fuel flow rate corresponds to 0.1 N chemical thrusters, and the plasma-assisted ignition is mainly attributed to thermal and kinetic effects. The temperature of the actuator and the plasma torch is investigated by infrared thermometry and spectral fitting of excited molecule nitrogen, respectively. First, the hot electrode tube helps evaporate water molecules and accelerate the liquid jet through rapid gas expansion. Then, the ionic liquid quickly decomposes to small species when the flow reaches the torch with a nitrogen vibrational temperature above 4000 K and rotational temperature above 2000 K. Meanwhile, the nonequilibrium plasma-excited species enhance the combustion of gaseous intermediates by direct impact dissociation, where the OH radical profile is visualized using the planar laser-induced fluorescence technique. In addition, the copper atoms released by plasma erosion are expected to exhibit a significant catalytic effect on ionic liquid decomposition, which is a critical step in controlling the overall reaction rate. It is also noted that such mild erosion does not affect the multi-start operations during the firing test that lasts for at least 30 min.

Cooling characteristics of hot rolled seamless steel tube by jet impingement

The flow and temperature field variation of hot rolled seamless steel tube with outer diameter of 140 mm and wall thickness of 10 mm were studied by experiment and numerical simulation. The finite element model of circular jet impinging cooling steel tube was established, and the steel tube was divided into different regions in circumferential direction. It was found that the uniformly arranged multiple jets have non-uniform cooling characteristics under the influence of gravity and hollow characteristic. Under the condition of 12 jets, the average heat transfer coefficient in circumferential direction of −15 ∼ −45° region is 8385 W/(m2K), and the fluctuation of the average heat transfer coefficient in different regions reaches 3222 W/(m2K). According to statistical average cooling rate of the experiment, the average cooling rate increases by 34% when 2 jets are increased to 4 jets. When the number of jets exceeds 8, increasing the number of jets has a limited effect on improving average cooling rate. Through reasonable configuration of jet velocity of different nozzle positions, a way to realize uniform cooling was proposed and preliminarily verified. The improved average heat transfer coefficient fluctuation of circumferential was limited to 1000 W/(m2K), which significantly improved the uniformity of jet cooling.

Effect of induction heating temperature on the microstructure and mechanical properties of HSLA square tubes

Severe strain hardening, thickness reduction, and large outer corner radius are three typical characteristics in the corner section of cold roll-formed steel. This paper presents a new manufacturing technology called cold and hot composite roll forming to improve the shape and properties in the corner section. In order to study the effect of induction heating temperature, a multi-pass cold and hot composite roll forming experiment for high-strength low-alloy steel (HSLA) square tubes was carried out under three different heating temperature ranges namely: the non-austenitizing zone, partial austenitizing zone, and austenitizing homogenizing zone. The microstructure and mechanical properties in the corner section of square tubes were studied using the material testing machine, optical microscope (OM), scanning electron microscope (SEM), and transmission electron microscope (TEM). Results showed that with the increase of induction heating temperature, the geometrical morphology and mechanical properties in the corner section of the cold and hot composite roll-formed square tube were significantly improved. Thus, compared with cold roll-formed square tubes, the corner thickness of cold and hot composite roll-formed square tubes was increased by 59%, the product of strength and elongation was increased by 48%, the outer corner radius was decreased by 91%, and no cracks appeared during the flattening test. Moreover, the proportion of granular bainite in the outer corner section increased, and the grain distortion in the inner corner section was relieved. The experimental results further showed that the austenitizing homogenizing zone was preferable among the three heating temperature ranges to manufacture high-quality HSLA square tubes.

Optimization design of process parameters for cold and hot composite roll forming of the AHSS square tube using response surface methodology

Advanced high-strength steel (AHSS) is a highly competitive material for the automobile industry to resolve the challenge of weight reduction and passenger safety. Cold and hot composite roll forming is a newly developed manufacturing technology, which aims to overcome the defects such as microcrack, severe work hardening, thickness reduction, and large corner radius in the traditional cold roll forming process of AHSS. In this paper, the mathematical models that represent the effect of line velocity, heating power, and deformation amount on the yield strength, outer corner radius, and microcrack length in the corner section of cold and hot composite roll-formed AHSS square tube are investigated by the response surface methodology with Box-Behnken design. The analysis of variance shows that the linear velocity has the greatest influence on the three responses, followed by the heating power and deformation amount. Furthermore, the experimental verification is carried out under the optimal combination of process parameters to manufacture the AHSS square tube with the desired performance. Compared to the cold roll-formed AHSS square tube, the product of strength and elongation in the corner section is increased by 53%, the corner thickness is increased by 60%, the outer corner radius is decreased by 94%, and the microcrack defect is not generated. The average magnitude of errors between the predicted and actual values is about 5%, which implies that the optimization design is accurate and reliable.

CFD simulation on optimum material to fabricate the counter flow of Ranque–Hilsch vortex tube

The basic objective of the current investigation by using CFD simulation is to select the appropriate material to fabricate Vortex tubes in meteorological conditions, which can improve efficiency and performance. The simulation results of vortex tubes were used in obtaining the performance for various materials such as copper, stainlessteel, brass, PVC, CPVC, acrylic, nylon, and bronze. The ratio of length of the hot tube and inner diameter was chosen as (L/D) 40 with the consistent inner diameter of the hot tube as 15 mm. Compressed air is passed through an inlet pipe between 2 and 12 bars in the step of 2 bars. 2D CFD was developed and analyzed with Ansys Fluent to access the performance of the vortex tube. Finally, after evaluating all simulation values for eight materials for performance depending upon their physical properties varies in the sequence from copper–bronze–brass–stainless steel–CPVC–PVC–acrylic–nylon material.

Investigation of thermal performance of a shell and tube latent heat thermal energy storage tank in the presence of different nano-enhanced PCMs

Thermal energy storage is crucial for the sustainability of an energy supply system. One of the best thermal energy storage systems is latent heat thermal energy storage tanks. In the present study, a shell and tube heat storage tank is simulated based on computational fluid dynamics. The tank consists of 18 hot and cold tubes and the space between them is filled with paraffin as a phase change material (PCM). The effects of adding SWCNT and GNP nanoparticles are evaluated on the PCM in different heat fluxes. The simulation is implemented by developing the C++ code in OpenFOAM software, and the solution is performed with the PIMPLE algorithm and finite volume method. Results reveal that the melting volume ratio of the PCMs is independent of the inlet heat flux, and increasing the inlet heat flux only accelerates the melting process. Adding SWCNT to the paraffin reduced the melting time by 43% and stored 147% more latent heat energy in comparison to pure paraffin.

ANALYSIS OF MASS FLOW RATE IN COLD WATER INTO FLOW WITH HOT WATER OUTPUT TEMPERATURE IN SHELL AND TUBE HEAT EXCHANGER

A heat exchanger is a device used to exchange heat and can usually function as a heater or a cooler. There are many types of heat exchangers available today. But the most widely used in industry is the shell and tube type heat exchanger. The shell and tube type heat exchanger has the advantages of being affordable, easy to maintain and has excellent heat transfer performance with a small volume. This paper aims to analyze the effect of mass flow rate on cold water inflow and hot water outlet temperature using water fluid in shell and tube heat exchanger. The design analysis that I did using the Solidworks 2019 software. This study only made a comparison by changing the mass flow velocity at the inflow of cold water with samples of 5 Kg/s, 10 Kg/s, 15 Kg/s, 20 Kg/s, 25 Kg/ s and 30 Kg/s and only sampled the hot water outlet temperature. The material used is stainless steel 321 on the shell and copper on the tube. The test results show that every increase of a multiple of 5 Kg/s there is a decrease in the average fluid temperature of 1.62 ℃ and the lowest temperature that can be achieved in this test is 52.33 ℃

Heat storage by melting the organic material of Paraffin RT50 in a heat exchanger with eccentric pipes

In this paper, the heat transfer inside a heat exchanger containing paraffin phase change material is investigated and the latent heat energy efficiency of RT50 material is calculated. Also, the effects of acentric condenser position on heat transfer and work efficiency are investigated in order to evaluate the energy efficiency by reducing the time required to melt the phase change material. On the other hand, in order to increase the thermal diffusivity, we have tried to improve the thermal conductivity by dispersion of SWCNT nanoparticles. Finally, the thermal analysis of the thermal energy storage system is performed by examining the effect of temperature changes in the condensers. The condensers are in the x and plus shapes, and the innovation of this study is the use of latent heat energy of phase change materials based on the passive technique. The results show that with increasing the distance of hot tubes from the center of the heat exchanger, the heat distribution increases, and the thermal efficiency increases by about 19 to 32 %. Also, by placing the hot tubes in the x-mode, the process of melting paraffinic materials leads to a reduction of time by 9 to 16 %.

Employing numerical method for evaluating the heat transfer rate of a hot tube by nanofluid natural convection

One of the major issues in industry is cooling of pipes when using forced convection mechanism is not able to cool pipes, but free convection mechanism can be a suitable method. In this regard, a half-pipe was installed in the center of a half-elliptical enclosure in which the gap of between them was saturated with nanofluid of water and alumina (aluminum oxide) nanoparticle depending on temperature and nanoparticle diameter. Several samples were simulated by means of CFD and FVM to understand the impacts of Rayleigh number, inclined angle, and volume fraction of nanoparticles on the heat flux of hot pipe. In addition, the dynamic viscosity and thermal conductivity coefficient of nanofluid depended on temperature, nanoparticle diameter, and volume fraction, which were used for precising simulation to physical results. The obtained results showed that adding 3% volume fraction of alumina could increase the average heat flux of hot half-pipe by 8.7%. Moreover, at high Rayleigh numbers, volume fraction of 1% was optimum amount for volume fraction. In addition, inclined angle had considerable influence on average heat flux, specifically, in high Rayleigh numbers. • Simulation of natural convection between a hot half-pipe and cold half-elliptical enclosure to cool hot pipe. • Dependence of nanofluid thermal conductivity and dynamic viscosity to temperature, its diameter, and volume fraction. • At high Rayleigh numbers, the optimum volume fraction is 0.01. • At R a = 10 3 a n d 10 4 , the optimal values of the inclined angle are 60 and 0°, respectively.

Convective heat transfer in I-shape heat sink under the action of Lorentz force via LBM

Via the non-orthogonal multiple-relaxation time Lattice Boltzmann Method, the natural convection phenomenon inside I-shape heat sink under the effect of Lorentz force is investigated. The cavity is filled with MWCNT-Fe3O4/water hybrid nanofluid, and three cold and three hot tubes exist inside the cavity at the same time. The influences of Rayleigh number (103–106), Hartmann number (0–100), magnetic field tilt angle θ (0 – π/2), and different cold and heat pipe arrangements (Case 1–Case 5) on the heat transfer and fluid flow are numerically simulated. The calculated results are shown by the variation of flow lines, isotherms and the average Nusselt number. The results show that the amplitude of the flow function is always the largest for Case 5, which is the best arrangement from the fluid motion point of view, while Case 2 has the largest Nusselt number for different Rayleigh numbers, magnetic field inclination angle θ and different Hartmann numbers, which is the best arrangement from the heat transfer efficiency point of view. However, with different cold and hot pipe arrangements, the average Nusselt number increases when the Rayleigh number and the magnetic field inclination angle θ increase. In contrast, as Hartmann number increases, the average Nusselt number decreases. The local Nusselt number of the cold and hot tubes is further investigated, comparing the discrepancy of the heat transfer performance near each wall surface.

Numerical modelling of Direct Hot Tube Rotary Draw Bending of 22MnB5 High Strength Steel

The use of High Strength Steel (HSS) tubular components in structural frames allows a drastic increase of the stiffness-to-weight ratio and, therefore, may open new possibilities to the design lightweight components for the transport industrial sector. However, the diffusion of these elements is still limited by the lack of manufacturing technologies capable of efficiently forming and bending HSS hollow profiles. To overcome the limits of traditional room temperature processes, an innovative rotary draw bending process carried out at high temperature is proposed, in which the profile cross section is quickly heated to the target temperature and, then, bent and selectively cooled to induce controlled microstructure changes. The paper presents a fully coupled thermo-mechanical-metallurgical numerical model to support the process design, which was applied to 22MnB5 tubes. The model was used to identify the optimal thermal and mechanical parameters to increase the material formability and to obtain the target microstructure in the final part. All the process steps, namely heating, handling, forming, and cooling, were modelled, and the results were experimentally validated by industrial trials carried out on a bending pilot plant. The results prove the possibility of an accurate prediction of both the final shape and the distribution of the microstructural and mechanical properties along the bend.

Simple model for flow field division and flow structure calculation in a vortex tube

The flow pattern and energy transfer process in the vortex tube are extremely complex, resulting in the difficulty of the direct calculation and prediction of the flow structure and temperature separation performance of a vortex tube. In view of this, research idea of dividing the vortex tube flow field into six regions was proposed, corresponding simplifications were made in different regions according to the flow features, and fluid parameters were coupled at the boundaries. An equation for the axial velocity component in the hot tube region was developed based on the characteristic that the axial velocity profiles under different conditions present similar patterns, the calculation method of reverse flow boundary which separates the working fluid into hot and cold fluids was described, and a set of relatively simple calculation methods for the internal flow field of vortex tubes was established based on the partition model. Three-dimensional velocity distributions inside the vortex tube under different working conditions were computed and analyzed. Further, the model was evaluated and validated through experimental data. The results showed good agreements of calculated data and experimental velocity fields. This work proposed a convenient calculation method for the estimation of the flow behavior inside a vortex tube and provide reliable predictions.

Numerical investigation on the effect of convergent-divergent tube on energy separation characteristic of vortex tube

The geometric structure of the hot end tube affects the energy separation of the vortex tube greatly. The computational fluid dynamics (CFD) technique is used to study the difference of velocity and temperature distribution between the straight tube and the convergent-divergent tube. The convergent-divergent angle varies within 3°-5°, and the results show that the temperature separation capability of the convergent-divergent structure performs better than the traditional straight tube. As the convergent-divergent angle increase, the temperature at the throat of the vortex tube gradually decreases and the velocity increases under a given cold mass fraction. At a high cold mass fraction, the increase of the convergent-divergent angle will increase the cooling capacity and heating capacity. The work-heat exchange theory is firstly applied to the convergent-divergent vortex tube. It is found that the tangential work and heat transfer of the vortex tube reaches maximum in case of convergent-divergent angle equals to 4° under a given cold mass fraction.