Mini Tube Research Articles

Investigation of Laminar Forced and Mixed Convection in Horizontal Mini Tubes with Uniform Wall Heat Flux Boundary Condition

In this study, a numerical model is established to study the forced and mixed convection heat transfer in the laminar region for horizontal mini tube with different diameters (1.2 mm to 3.0 mm) under the uniform wall heat flux boundary condition. The Reynolds number is set from 100 to 2,000 and the uniform wall heat flux boundary condition is set at 10 and 20 kW/m2. The model is verified with well-established correlation to guarantee good accuracy. The existence of mixed convection is examined by (1) the ratio between the heat transfer coefficients at the top and the bottom of the tube, and (2) the temperature contour plot. Based on the simulation results, the existence of buoyancy superimposed on the main flow is strongly related to the tube diameter, the dimensionless location, the Reynolds number, and the heat flux applied. Under the same heating condition, for small diameter tube with insufficient length, buoyancy effect can never establish. The existing flow regime maps for macro tube were used to predict the forced and mixed convection regimes from the simulated mini tube results and discrepancy was found. A new flow regime map is hence developed for mini tubes, and it classified the simulation results with good accuracy.

Transient simulation of conjugate heat transfer of kerosene with thermal oxidative and surface coking reactions at a supercritical pressure

The regenerative cooling technology in aerospace applications often involves supercritical-pressure heat transfer of a hydrocarbon fuel, in which thermal oxidative reactions of the fuel would lead to surface coke accumulation on the cooling channel wall. In this paper, the long-term transient conjugate heat transfer of kerosene has been numerically investigated in a circular mini tube at a supercritical pressure of 3 MPa. The modified chemical mechanism and kinetics of thermal oxidative and surface coking reactions of kerosene are proposed, which, with an additional surface reaction, are capable of treating large coke deposition in regions with sharp temperature gradients. Transient surface coke accumulation and its effect on heat transfer are handled by a dynamic mesh technique. A series of numerical studies are conducted to elucidate the influences of surface heat flux, dissolved oxygen mass fraction, inlet fuel mass flow rate, and coke layer thermal conductivity on the supercritical-pressure heat transfer and thermal oxidative reactions. The effects of different reaction pathways on surface coke accumulation are analyzed comprehensively, and it is revealed that the oxygen-consuming surface coke deposition is initiated only at the high fuel temperature at around 590 K. The present numerical results would enhance fundamental understanding of the long-term transient heat transfer process of a hydrocarbon fuel at supercritical pressures in the regenerative cooling applications.

Visualization of the Two-Phase Flow Behavior Involved in Enhanced Dense Phase Carbon Dioxide Pasteurization by Means of High-Speed Imaging

This research explores the two-phase flow behavior involved in enhanced dense phase carbon dioxide inactivation of E. coli DH5α, which has been shown to possess a high microbial reduction efficiency of up to 3.7 ± 0.4 log. We present an experiment in which the liquid sample was pressurized with liquid carbon dioxide to 8.2 MPa and, after saturation, was forced to flow through a mini tube. An experimental setup was developed to visualize the flow patterns (plug, slug and churn flows) occurring in the mini tube by means of high-speed imaging. The values of the wall shear stress were estimated within the mini tube with the help of the gas slug velocities (8–9 m/s) and were compared with threshold shear stress values reported for the disruption of fresh E. coli cells. The results suggest that the preliminary pressurization phase may cause a substantial destabilization of the cell wall of E. coli DH5α.

Effect of surfactants on the convective heat transfer and pressure drop characteristics of ZnO/DIW nanofluids: An experimental study

The advancement of nanotechnology has demonstrated the ability of metal-oxide-based nanofluids (NFs) to produce high heat flux in microscale thermal applications. Convective heat transfer (HTC) and flow characteristics (pressure drop (ΔP) and friction factor (f)) of aqueous ZnO NFs' within a circular mini tube (Di = 1.0 mm, L = 330 mm) were analyzed. Experiments were carried out under steady-state and varying flow rates using 0.012–0.048 wt % of NFs and sodium hexametaphosphate (SHMP) and acetylacetone (ACAC) as surfactants (SFs). Laminar flow and constant wall heat flux conditions were used to assess NFs heat transfer properties, ΔP and f. The viscosity (VC) and thermal conductivity (TC) of NFs exhibited a strong dependence on the operating temperature and NFs concentration. VC and TC increased by increasing the NFs concentration and decreased by increasing the operating temperature. Maximum VC and TC enhancement of 16.75% and 23.70% were achieved for SHMP-stabilised NFs, respectively. The average HTC increased by increasing NFs loading and flow rate, with HTCmax of 17.0% noticed for ACAC-stabilised NFs. The ΔPmax and fmax were 16.0% and 12.0%, respectively. Experimental and theoretical results showed a maximum deviation of ±7.0% and ±4.0%, respectively.

Numerical study on flow and heat transfer characteristics of R1233zd(E) cooled in horizontal circular tubes under supercritical pressure

Supercritical organic refrigerants have been widely used in energy conversion and waste heat recovery systems due to their excellent heat transfer characteristics. R1233zd(E), as a fourth-generation environmentally friendly alternative refrigerant, has extensive application prospects. However, there is a lack of research on the heat transfer of supercritical R1233zd(E). In this work, the flow and heat transfer characteristics of supercritical R1233zd(E) in horizontal cooling tubes, including a mini tube and three ordinary tubes, are numerically simulated. In the simulation, not only the wall thickness but also the temperature-dependent thermophysical properties of the tube wall material are considered. The simulated results indicate that the specific heat capacity distributions determine that the peak of the overall heat transfer coefficients appears at a temperature higher than the pseudo-critical temperature. The flow and heat transfer characteristics of supercritical R1233zd(E) in different tubes, especially in the mini tube, are different. The applicability of three typical buoyancy criteria to this study is discussed, and the results show that two of three typical buoyancy criteria can be used to predict the buoyancy effect of supercritical R1233zd(E) in this study, but the thresholds of mini tube and ordinary tubes are different. Finally, a new heat transfer correlation is developed for supercritical R1233zd(E) by using the piecewise fitting method with pseudo-critical temperature as the point of division. The average relative deviation of the correlation is 0.57 % and the root mean square deviation is 0.87 %.

Experimental investigations of gas–liquid two-phase flow in a horizontal mini pipe: Flow regime, void friction, and frictional pressure drops

Experimental studies of air–water two-phase flows in a mini tube with an inner diameter of 3.12 mm have been conducted in terms of the flow pattern, void fraction, and pressure gradients. The fluid velocities range from 0.065 to 21.78 m/s and from 0.109 to 1.835 m/s, respectively. A right-angle prism is applied to capture images simultaneously from two perpendicular directions. Three-dimensional gas–liquid interfaces are reconstructed with the obtained images from two directions. The gas void fraction is investigated using this method, which is experimentally validated with the quick closing valves method. Three flow patterns are obtained in the present work, which is bubbly, slug, and annular flows. The Probability Density Functions of the cross-sectional void fraction show that the intermittent flow has two peaks and the annular flow has only one peak. Volumetric void fraction is affected by the flow pattern and the flow rate. However, at very high gas velocities, volumetric void fraction is independent of the liquid rate. A new frictional pressure drop correlation is proposed based on the experimental data.

Fabrication and Evaluation of a Flexible MEMS-Based Microthermal Flow Sensor.

Based on the results of computational fluid dynamics simulations, this study designed and fabricated a flexible thermal-type micro flow sensor comprising one microheater and two thermistors using a micro-electromechanical system (MEMS) process on a flexible polyimide film. The thermistors were connected to a Wheatstone bridge circuit, and the resistance difference between the thermistors resulting from the generation of a flow was converted into an output voltage signal using LabVIEW software. A mini tube flow test was conducted to demonstrate the sensor’s detection of fluid velocity in gas and liquid flows. A good correlation was found between the experimental results and the simulation data. However, the results for the gas and liquid flows differed in that for gas, the output voltage increased with the fluid’s velocity and decreased against the liquid’s flow velocity. This study’s MEMS-based flexible microthermal flow sensor achieved a resolution of 1.1 cm/s in a liquid flow and 0.64 cm/s in a gas flow, respectively, within a fluid flow velocity range of 0–40 cm/s. The sensor is suitable for many applications; however, with some adaptations to its electrical packaging, it will be particularly suitable for detecting biosignals in healthcare applications, including measuring respiration and body fluids.

Numerical Investigation on Convective Heat Transfer of Supercritical Carbon Dioxide in a Mini Tube Considering Entrance Effect

Numerical Investigation on Convective Heat Transfer of Supercritical Carbon Dioxide in a Mini Tube Considering Entrance Effect

Are all blood-based postal sampling kits the same? A comparative service evaluation of the performance of dried blood spot and mini tube sample collection systems for postal HIV and syphilis testing

ObjectivesWe comparatively evaluated two HIV and syphilis blood sampling kits (dried blood spot (DBS) and mini tube (MT)) as part of an online STI postal sampling service that included tests...

Numerical study on the heat transfer deterioration and its mitigations for supercritical CO2 flowing in a horizontal miniature tube

Responding to the rapid growth of size reduction in engineering systems, it is crucial to explore the safety performance of supercritical fluid systems in miniature tubes. In this paper, numerical studies are conducted to explore the performance of baffles on heat transfer deterioration (HTD) mitigations for supercritical CO2 (sCO2) flowing in a horizontal mini tube using RNG k-e turbulence model with enhanced wall treatment (EWT). Three kinds of baffle arrangements: in-lined, staggered and centred, with the same blockage ratio (BR = 0.25) are selected to explore their performance on HTD mitigation. Thethermo-hydraulic performances of the baffled tubes are examined using the dimensionless parameters based on normalized Nusselt numbers and Fanning friction factors,Nu/Nu0,f/f0andPEC=(Nu/Nu0)/(f/f0)1/3. The results reveal that unlike the in-lined and staggered baffles, the centred baffles induce jet impingements directly onto the tube walls and generate significant amounts of transverse fluid velocity near the tube wall (which exceeds that in the core region),making it the most influential baffle arrangement on the HTD mitigations. The effects of baffles on buoyancy flow under a wide range of heat and mass fluxes are explored based on Jackson’s criterion (Gr/Re2<103). It is observed that the Gr/Re2 increases at a relatively lower rate in baffled tube than in the smooth tube under increased heat flux variation whereas it decreases at a higher rate than the smooth tube under increased mass flux variation, thus revealing the buoyancy weakening effects of baffles. Finally, the baffle’s performance on HTD mitigation when the tube is further miniaturized is explored and the results unveil that miniaturization weakens buoyancy, and there exists a certain mini size (D = 0.5 mm) at which buoyancy influence completely vanishes, and the effect of baffles on HTD mitigation becomes insignificant.

Experimental study of air-water two-phase flow pattern evolution in a mini tube: Influence of tube orientation with respect to gravity

The evolution of two-phase flow patterns is investigated experimentally in a 1500 mm long capillary tube of 3 mm inner diameter with different orientations with respect to gravity. The flow patterns are recorded with a fast camera (1000 fps) at two locations along the tube under the same operating conditions. The first is close to the inlet zone at L/D = 10 and the second is far downstream at L/D = 420. Co-flowing configurations downward and upward are considered where air is injected through a nozzle of 110 μm of inner diameter and centered on the tube axis. Unlike many existing studies in the literature, the present work explores ranges of small superficial velocities of the two fluids, from 0.78 × 10−3 m/s to 79 × 10−3 m/s for the liquid phase and from 2.3 × 10−3 m/s to 3.54 m/s for the gas phase, in order to deal with smooth interfaces. The experiments show that the flow patterns are very sensitive to inlet conditions. Although, the superficial velocities considered are very small, several two-phase flow patterns are visualized namely bubbly flow, slug flow, unstable annular and annular flow. In the upward flow, the annular pattern is unstable, while for the downward flow, the bubbly pattern is difficult to obtain. Phenomena like coalescence and breakup yield to the evolution of the flow pattern from the entrance of the tube to the downstream location. The visualizations highlight also that certain flow patterns evolve from one structure to another along the tube from the location L/D = 10 to L/D = 420. Furthermore, the two-phase flow maps are drawn and the effect of gravity on the flow maps as well as on the flow pattern transitions is discussed.

Numerical studies of gas-liquid Taylor flows in vertical capillaries using CuO/water nanofluids

Heat transfer and pressure drop characteristics of gas-liquid Taylor flows in a mini tube with 1 mm inner tube diameter were investigated numerically using a moving frame of reference method. A CuO/water nanofluid was used as the continuous phase, while nitrogen was adopted as the dispersed phase. The inlet Reynolds number ranged from 250 to 600, and the volume concentration of the CuO particles was in the range of 0% to 3%. The results show that a thicker liquid film and a relatively longer bubble are obtained for Taylor flows with nanofluids compared with those using pure water. The heat transfer process could be divided into three stages with increasing time. At the initial stage, a quick increase of the thermal boundary layer results in a dramatic decrease of the heat transfer coefficient. With increasing time, heat transfer coefficient oscillation is obtained because of the advection of cold liquid from the tube center to the heated wall. With the combined effect of thermal diffusion and recirculation in liquid slugs, the fully developed status of Taylor flow is obtained. Heat transfer coefficients increase with decreasing gas void fraction and with increasing nanoparticle concentration. The overall two-phase pressure gradients increase with increasing nanoparticle concentration and Re, but with decreasing gas void fraction. The increase in the thermal conductivity and the viscosity of nanofluids is the main reason for heat transfer enhancement and pressure drop penalty.

Investigating on the Iconic Gas Compositions Produced by Low-Temperature Heating Cotton

This study aims at achieving the generation laws of the iconic gas compositions produced during the process of the cotton smoldering. A mini tube furnace was used to heat a long-staple cotton sample from Xinjiang, China, and a gas chromatography-mass spectrometer (GC/MS) was applied to analyze the produced organic and inorganic gas composition at different low temperatures ranging from 95 °C to 185 °C. Besides, a thermogravimetric experiment under air atmosphere was done in order to verify the correctness of the inferences. The pathways of product generation by heating cotton were set forth, including the depolymerization of the cotton cellulose, the sequential generation of coke, low molecular weight products, long chain carboxylic acids, levoglucosenone, and acetone at 125 °C, and the generation of hydrogen at 95 °C, carbon dioxide, carbon monoxide at 145 °C, and methane during the heating process. The results showed that the alkanes, furans, alkenes, aldehydes, hydrazines, and acids could not be regarded as the iconic gas compositions because of their little proportion, the joint detection of the methane and hydrogen could be used to predicate the smoldering, and the acetone and carbon monoxide could be used to confirm the smoldering stage. The results of the thermogravimetric experiment are consistent with theoretical analysis, indicating that analyses are reliable. This work will provide significant practice foundation for the early warning and prevention of cotton fires.

Experimental study on heat transfer of supercritical CO2 flowing in a mini tube under heating conditions

Supercritical carbon dioxide (S-CO2) power cycles hold great potential applications in various energy utilization systems, in which the heat transfer of S-CO2 is one of the most important issues. However, due to the complex thermophysical properties, it is still insufficient to predict heat transfer coefficient of S-CO2, especially under the heating conditions with high ratios of heat flux to mass flux (q/G). In this study, the heat transfer of S-CO2 is experimentally investigated in a mini tube (d = 2 mm) in the range of p = 7.6–8.4 MPa, q = 100–200 kW/m2 and G = 400–700 kg/m2 s, in which the ratio of q/G is within 250–500 J/kg. The parameter influences on heat transfer coefficient of S-CO2 are analyzed. It is found that the heat transfer coefficient decreases with increasing heat flux and decreasing mass flux, but is little affected by pressure. Under high q/G conditions, buoyancy effect is significant and heat transfer is deteriorated in the whole test section. Available heat transfer correlations for S-CO2 are compared with the experimental data; however, the prediction errors of these correlations are relatively high, especially in pseudo-critical regions. As a result, a new heat transfer correlation is developed for S-CO2 under high q/G conditions, in which buoyancy effect and variations in thermophysical properties are both taken into account.

Experiments on Heat Transfer of Supercritical Pressure Kerosene in Mini Tube under Ultra-High Heat Fluxes

Heat transfer of supercritical-pressure kerosene is crucial for regenerative cooling systems in rocket engines. In this study, experiments were devoted to measure the heat transfer of supercritical-pressure kerosene under ultra-high heat fluxes. The kerosene flowed horizontally in a mini circular tube with a 1.0 mm inner diameter and was heated uniformly under pressures of 10–25 MPa, mass fluxes of 8600–51,600 kg/m2 s, and a maximum heat flux of up to 33.6 MW/m2. The effects of the operating parameters on the heat transfer of supercritical-pressure kerosene were discussed. It was observed that the heat transfer coefficient of kerosene increases at a higher mass flux and inlet bulk temperature, but is little affected by pressure. The heat transfer of supercritical-pressure kerosene is classified into two regions: normal heat transfer and enhanced heat transfer. When the wall temperature exceeds a certain value, heat transfer is enhanced, which could be attributed to pseudo boiling. This phenomenon is more likely to occur under higher heat flux and lower mass flux conditions. In addition, the experimental data were compared with several existing heat transfer correlations, in which one of these correlations can relatively well predict the heat transfer of supercritical-pressure kerosene. The results drawn from this study could be beneficial to the regenerative cooling technology for rocket engines.

Experimental investigation on convection heat transfer of supercritical hydrocarbon fuel in a long mini tube

Thermal management system is crucial for hypersonic vehicles, in which a large amount of thermal energy due to aerodynamic heating needs to be removed by hydrocarbon fuel. In this study, an experimental investigation is performed on the convection heat transfer of supercritical hydrocarbon fuel in a long mini tube. The horizontal test tube has an inner diameter of 1.0 mm and a length-to-diameter ratio of 980, which are close to those of practical heat-transfer channels in the thermal management system for hypersonic vehicles. The non-linear distribution characteristics of wall temperature and heat transfer coefficient along the test tube are presented. The heat transfer regions of hydrocarbon fuel are classified into initial heat transfer (IHT), normal heat transfer (NHT), enhanced heat transfer (EHT) and deteriorated heat transfer (DHT). The heat transfer mechanisms in each heat transfer region are discussed in detail. The heat transfer of hydrocarbon fuel are affected by the dramatic variations in thermo-physical properties and buoyancy effect. Heat transfer is commonly deteriorated under disadvantageous thermo-physical properties and absence of buoyancy effect. The results could be beneficial for the analysis and design of the thermal management system for hypersonic vehicles.

Effect of Various Evaluation Criteria on Heat Transfer Enhancement of Nanofluids: A Case Study of Water-Based Cu2O Nanofluids

The numerical study investigates the effect of various comparison criteria on the forced convective heat transfer of water-based cuprous oxide nanofluids in a horizontal mini tube under uniform heat flux conditions. The forced convective flow regime was validated, and it was ensured that the free convection effects were not present. The temperature-dependent thermophysical properties were considered for the numerical simulation. The volume concentrations of the nanofluids were kept below 2% (i.e., 0.6%, 1.3% and 1.9%) as higher concentrations lead to agglomeration and sedimentation of nanoparticles. The various comparison criteria employed for the assessment of heat transfer enhancement were equal pumping power, equal velocity, equal mass flow rate and equal Reynolds number criterion. The enhancements in heat transfer coefficient (at ϕv = 1.9% and base fluid Re = 1380) were 7.7%, 4.9%, 3.6% and 3.3% when assessed on the basis of equal Reynolds number, equal velocity, equal mass flow rate and equal pumping power, respectively. It was found that the enhanced heat transfer of the nanofluids should preferably be examined on the basis of equal pumping power criterion instead of above-mentioned other three criteria. The discussion on the invalidity of three evaluation criteria based on Reynolds number, velocity and mass flow rate has also been made.

Experimental investigation on subcooled boiling heat transfer of emulsified kerosene

To investigate heat transfer characteristics of emulsified kerosene in a mini tube, experiments were conducted at the pressure of 3.0–5.0 MPa, mass flow rate of 0.8–2.4 g·s−1, and water mass fraction of 10%–50%. Water in oil type emulsion was prepared using the surfactant of span-80. The basic heat transfer characteristics of emulsified kerosene covering a wide temperature range were revealed. More attention was focused on subcooled boiling of emulsified kerosene. Depending mainly on low-boiling temperature dispersed phase of water, the heat transfer of emulsified kerosene could be divided into 6 regions: single liquid-phase convection, subcooled boiling, saturated nucleate boiling, film boiling, steam-kerosene convection, and single gas-phase convection. Heat transfer deterioration characterized by a significant soar in wall temperature could be observed in the film boiling region. In the fully developed subcooled boiling region, higher heat transfer coefficient could be achieved at lower pressure. With an increase in mass flow rate, subcooled boiling was postponed to higher heat flux. At the same heat flux, the fluid temperature was negatively correlated with the water mass fraction. However, the effect of water mass fraction on inner wall temperature could be ignored. The subcooled boiling heat transfer was enhanced at lower water mass fraction. Most of the existing enhancement-factor type correlations underestimated the subcooled boiling heat transfer of emulsified kerosene. According to the experimental data, a new enhancement-factor type correlation was proposed for subcooled boiling heat transfer of emulsified kerosene. The mean absolute deviation of the new correlation was 9.9%.

The miniaturised process for lead removal from water samples using novel bioconjugated sorbents

ABSTRACT In the present study, novel bioconjugated sorbents (peptide and oligo-nucleotide on the cobalt aluminate nanomaterials) were used to remove lead through miniaturised process and the concentration of lead was measured by inductively coupled plasma mass spectrometry. For this aim, lead was collected on sorbent in mini tube, and the influences of experimental conditions (e.g. pH of sample, amount of sorbent, concentration of eluent, foreign ions) and retention parameters on the recovery of the lead element were examined. After the optimisation of experimental parameters, a successful separation was obtained at pH 7.5 with high (>95%) quantitative recovery and high precision (<10% relative standard deviation). Using the proposed bioconjugated sorbents, the lead in sea water and tap water samples could be practically and easily removed with 95% confidence level. The detection limits of this method for lead using oligonucleotide and peptide on the cobalt aluminate nanomaterials were 0.14 and 0.12 µg/L (3σ, N = 10) with sample-matched blanks. This method can be widely used as promising and cost-effective nanomaterials to remove lead from water systems.

Dried blood spot and mini-tube blood sample collection kits for postal HIV testing services: a comparative review of successes in a real-world setting

ObjectivesThis is a comparative review between using dried blood spot (DBS) and mini-tube (MT) HIV sampling kits as part of an online sexually transmitted infection (STI) postal testing service. England...