Heat transfer,friction factor and effectiveness analysis of Fe3O4/water nanofluid flow in a double pipe heat exchanger with return bend

The convective heat transfer, friction factor and effectiveness of different volume concentrations of Fe₃O₄ nanofluid flow in an inner tube of double pipe heat exchanger with return bend has been estimated experimentally and turbulent flow conditions. The test section used in this study is of double pipe type in which the inner tube diameter is 0.019 m, the annulus tube diameter is 0.05 m and the total length of inner tube is 5 m. At a distance of 2.2 m from the inlet of the inner tube the return bend is provided. The hot Fe₃O₄ nanofluid flows through an inner tube, where as the cold water flows through an annulus tube. The volume concentrations of the nanoparticles used in this study are 0.005%, 0.01%, 0.03% and 0.06% with Reynolds number range from 15,000 to 30,000. Based on the results, the Nusselt number enhancement is 14.7% for 0.06% volume concentration of nanofluid flow in an inner tube of heat exchanger at a Reynolds number of 30,000 when compared to base fluid data; the pumping penalty of nanofluid is < 10%. The effectiveness of heat exchanger for water and nanofluid flow is explained in terms of number of transfer units (NTU) in order to estimate the overall performance of the double pipe heat exchanger. New correlations for Nusselt number and friction factor have been developed based on the experimental data.     

Experimental investigation on two-phase flow boiling heat transfer of five refrigerants in horizontal small tubes of 0.5, 1.5 and 3.0 mm inner diameters

An experimental investigation on two-phase flow boiling heat transfer with refrigerants of R-22, R-134a, R-410A, C₃H₈ and CO₂ in horizontal circular small tubes is presented. The experimental data were obtained over a heat flux range of 5–40 kW m^?2, mass flux range of 50–600 kg m^?2 s^?1, saturation temperature range of 0–15 °C, and quality up to 1.0. The test section was made of stainless steel tubes with inner diameters of 0.5, 1.5 and 3.0 mm, and lengths of 330, 1000, 1500, 2000 and 3000 mm. The experimental data were mapped on Wang et al. (1997) [5] and Wojtan et al. (2005) [6] flow pattern maps. The effects of mass flux, heat flux, saturation temperature and inner tube diameter on the heat transfer coefficient are reported. The experimental heat transfer coefficients were compared with some existing correlations. A new boiling heat transfer coefficient correlation that is based on a superposition model for refrigerants in small tubes is presented with 15.28% mean deviation and ?0.48% average deviation.     

Air heat exchangers with long heat pipes: Experiments and predictions

This paper presents measurements and predictions of a heat pipe-equipped heat exchanger with two filling ratios of R134a, 19% and 59%. The length of the heat pipe, or rather thermosyphon, is long (1.5 m) as compared to its diameter (16 mm). The airflow rate varied from 0.4 to 2.0 kg/s. The temperatures at the evaporator side of the heat pipe varied from 40 to 70 °C and at the condenser part from 20 to 50 °C. The measured performance of the heat pipe has been compared with predictions of two pool boiling models and two filmwise condensation models. A good agreement is found. This study demonstrates that a heat pipe equipped heat exchanger is a good alternative for air–air exchangers in process conditions when air–water cooling is impossible, typically in warmer countries.     

Single-phase heat transfer in the high temperature multiple porous insulation

An experimental study of steady state flow and heat transfer has been conducted for the multiple plate porous insulation used in the reactor pressure vessels of ‘Magnox’ nuclear power stations. The insulation pack studied, consisting of seven dimpled stainless steel sheets and six plane stainless steel sheets, was of the type installed in the Sizewell A plant. A large scale experimental test facility, based on the guarded hot plate method, was used for measuring the effective thermal conductivity of Magnox reactor pressure vessel insulation, which consists of alternate layers of plain steel foil and dimpled foil. The measurements were made both with the fluid within the insulation pack nominally stationary and with an imposed flow through it, simulating leakage through the insulation pack. The experimental conditions corresponded to a heat flux of 75–1000 W/m², fluid pressures of atmospheric to 5 bar gauge, pack orientations in range of 0°–45° relative to the horizontal, leakage velocities ranging from 0.05 m/s to 0.20 m/s and inlet air bulk temperatures ranging from 18 °C to 290 °C. Local values of effective thermal conductivity of 0.04–0.23 W/m K were obtained for the above experimental conditions. The heat transfer modes in the insulation pack were conduction through the contacting metallic foils, thermal radiation across the gas gaps, and conduction and convection in the air. The effective thermal conductivity of the porous insulation increased with increasing air pressure, inclination angle, and air velocity. Buoyancy effects increased with increasing inclination angle and air pressure.     

Simulation of heat transfer to separation Air flow in a concentric pipe

Flow separations occur in various engineering applications. Computational simulation by using standard k-ε turbulence model was performed to investigate numerically the characteristic of backward-facing step flow in a concentric configuration. This research is focused on the variation of Reynolds number, heat flux and step height in a fully developed turbulent air flow. The design consists of entrance tube, and inner and outer tubes at the test section. The inner tube is placed along the entrance tube at the test section with an outer tube to form annular conduit. The entrance tube diameter was varied to create step height, s of 18.5 mm. The Reynolds number was set between 17,050 and 44,545 and heat flux was set between 719 W/m² and 2098 W/m² respectively. It is observed that the higher Reynolds number with step flow contributes to the enhancement of heat transfer. The reattachment point for q = 719 W/m² is observed at 0.542 m, which is the minimum surface temperature. The experimental data shows slightly lower distribution of surface temperature compared to simulation data. As for the same case in experimental result, the minimum surface temperature is obtained at 0.55 m. The difference between numerical and experimental result is 0.008 m. Finally, it can be inferred that utilizing the computational fluid dynamic package software, agreeable results could be obtained for the present research.     

Assessing the influence of four bonding methods on the thermal contact resistance of open-cell aluminum foam

A thermal application of open-cell aluminum foam typically requires it to be bonded on a substrate. The resulting thermal contact resistance is investigated for four bonding methods. This is done by minimizing the difference between the calculated heat transfer via a zeroth order model and experimental data. The bonded metal foam, used to obtain the experimental data, are manufactured in-house. This allows varying pore size, porosity, aluminum alloy, foam height, air mass flow rate, air inlet temperature and bonding method. The latter is found to have an overwhelming impact. The resulting four thermal contact resistances are: 0.7 × 10^?3 m²K/W for brazing, 0.88 × 10^?3 m²K/W for co-casting, 1.25 × 10^?3 m²K/W for a single-epoxy bonding and 1.88 × 10^?3 m²K/W for a press-fit bonding. The uncertainty on these values is 11%.     

Effect of louvres on the thermal efficiency of panel radiators

In this study, two radiators on which louvres had been made up on either outer surface or inner surface, were experimentally contrasted within an experimental setup which heats water between an average temperature difference range of ΔT = 30 K to 60 K. Constructive details and dimensions of experimental test section's orientations having 6 different positions were given in detail in the paper. Positions include the situation of the panel radiator without louvre, louvres on front surface-single slice, louvres on back surface-single slice, louvres on back surface-single slice, and louvers on back surface-single slice with a chimney, louvres on back surface- 3 slices in a row. Detailed uncertainty analyses were given in order to calculate the uncertainties of water inlet and outlet temperatures, and heat output values. CFD analyses have been carried out to have air velocity and temperature distribution around the sample panel radiators after the validation with experiments. CFD results were found to be in good agreement with experiments having tolerable deviation varied from 10.73% to 24%. Almost all experimental findings including measured data points were declared in a Table for other researchers' usage for their validation issues. 8 number of empirical correlations have been proposed in order to determine the heat outputs regarding with the different configurations of the test section. Finally, effect of louver space on the heat outputs was also shown in the paper.     

Two-phase pressure drops in a helically coiled steam generator

An experimental investigation regarding two-phase diabatic pressure drops inside a helically coiled heat exchanger have been carried out at SIET thermo-hydraulics labs in Piacenza (Italy). The experimental campaign is part of a wide program of study of the IRIS innovative reactor steam generator. The test section consists of an AISI 316 stainless steel tube, 32 m length, 12.53 mm inner diameter, curved in helical shape with a bend radius of 0.5 m and a helix pitch of 0.8 m, resulting in a total height of the steam generator tube of 8 m. The explored operating conditions for two-phase flow experiences range from 192 to 824 kg/m² s for the mass flux, from 0 to 1 for the quality, from 1.1 to 6.3 MPa for the pressure, from 50 to 200 kW/m² for the heat fluxes. A frictional two-phase pressure drops correlation, based on an energy balance of the two-phase mixture and including the 940 experimental points, is proposed. Comparison with existing correlations shows the difficulty in predicting two-phase pressure drops in helical coil steam generators.     

Creep constitutive equations for a 0.5Cr 0.5 Mo 0.25V ferritic steel in the temperature range 565 °C–675 °C

A two damage state variable model is used to describe the softening mechanisms, damage initiation and growth for a low alloy ferritic steel 1/2Cr–1/2Mo–1/4V at 565 and 590 °C within the Continuum Damage Mechanics framework. The level of complexity of the constitutive equations and the degree of coupling through damage is high and it is difficult to calibrate values of the constitutive constants without recourse to optimisation techniques. A methodology for the analysis of uni-axial experimental data, coupled with a traditional gradient-based optimisation technique, is presented for the unique determination of the constitutive constants. Two sets of experimental data on parent material are used for inversion purposes: at 565 °C, c.f. Cane [Cane BJ. Collaborative programme on the corelation of test data for high temperature design of welded steam pipes. Presentation and analysis of the material data. Note No. RD/L/2101N81, March, CEGB Laboratory; 1981]; and, at 590 °C, c.f. Miller [Miller DA. Private communication: “Creep rupture testing of Cr M V pipe steel. ERA Project 63-01-040320009”. Barnwood, Gloucs, UK: British Energy; 2000]. The variation of the constitutive parameters with temperature in the range 565–590 °C has been deduced by considering the values of constitutive parameters for the same alloy deduced by Perrin and Hayhurst [Perrin IJ, Hayhurst DR. Creep constitutive equations for a 0.5Cr–0.5Mo–0.25V ferritic steel in the temperature range 600–675 °C. J Strain Anal 1996;31:299–314] in the temperature range 620–675 °C.     

Heat transfer performance in 3D internally finned heat pipe

An experimental study of heat transfer performance in 3D internally finned steel-water heat pipe was carried out in this project. All the main parameters that can significantly influence the heat transfer performance of heat pipe, such as working temperature, heat flux, inclination angle, working fluid fill ratio (defined by the evaporation volume), have been examined. Within the experimental conditions (working temperature 40 °C–95 °C, heat flux 5.0 kw/m²–40 kw/m², inclination angle 2–90°), the evaporation and condensation heat transfer coefficients in 3D internally finned heat pipe are found to be increased by 50–100% and 100–200%, respectively, as compared to the smooth gravity-assisted heat pipe under the same conditions. Therefore, it is concluded that the special structures of 3D-fins on the inner wall can significantly reduce the internal thermal resistance of heat pipe and then greatly enhance its heat transfer performance.     

A mathematical model for predicting the performance of a compound desiccant wheel (A model of compound desiccant wheel)

A mathematical model for predicting the performance of novel silica gel haloid compound desiccant wheel is established. Both the gas side resistance and the solid side resistance are considered in the model. It is found that the results of this model agree better to the experiments than the results of a former model which does not take the solid side resistance into account at all. Then the model is adopted to analyze the effects of some main parameters on system performance. It is found that the compound desiccant wheel has a better performance in a climate with moderate temperature or in a climate with high humidity ratio. Then under the basic conditions for the simulations (ambient air is of 35 °C, 15 g/kg and wheel thickness of 100 mm), an angle of the regeneration section between 100° and 160°, a regeneration temperature between 80 °C and 95 °C, a flow rate of process air between 2.0 m/s and 3.5 m/s and that of regeneration air between 2.5 m/s and 3.5 m/s are recommended. Also, there exists an optimal rotation speed to achieve the maximal moisture removal, which is about 12 r/h. At last, the influences of the main parameters on optimal rotation speed are discussed.     

Experimental study of conjugate heat transfer within a bottom heated vertical concentric cylindrical enclosure

In this research work an experimental study of conjugate heat transfer within an air filled bottom-heated vertical enclosure is conducted. The enclosure consists of two concentric cylinders with inner cylinder being shorter and open at the top. The study is important with respect to the centrifuge machine used in the process industry. Eighteen different experiments are performed by varying the bottom disc central temperature between 353 and 433 K, using three different materials (aluminum, mild steel and stainless steel) of the inner cylinder and two different diameter outer cylinders of mild steel. This study unfolds the temperature, material and geometric effects of bottom disc, inner cylinder and outer cylinder respectively on thermal convection in the enclosure. Generally, a uniform temperature is required in such enclosures. A more uniform axial and radial temperature is observed in the enclosure by using aluminum inner cylinder within a temperature range of 353–433 K of the bottom disc and using two different diameter outer cylinders. It is observed that the maximum temperature in the enclosure is lowest for aluminum inner cylinder and higher for mild steel and highest for stainless steel. The heat balance and non-dimensional analysis of the enclosure are carried out and discussed critically.     

Dispersion in the near wake of idealized car model

The paper presents some recent experimental results concerning tracer gas diffusion within the near wake of a simplified model car. The model car is an Ahmed model (with a rear slant angle of 5°, 25° or 40°). Pollutant emission is simulated using heated air injected through a small pipe at one side of the model base. Fine cold wire thermometry is used to measure instantaneous temperature excess in the near wake. Characteristics of the mean and fluctuating temperature fields are presented for the near wake (0 < x¹ < 10) where x¹ is the distance x downstream from the model normalized by the model height H. Results are explained accounting for the velocity field.     

Experimental analysis and FEM simulation of finned U-shape multi heat pipe for desktop PC cooling

This paper presents the performance analysis of a finned U-shape heat pipe used for desktop PC-CPU cooling. The experiments are conducted by mounting the system vertically over a heat source situated inside a rectangular tunnel, and force convection is facilitated by means of a blower. The total thermal resistance (R_t) and heat transfer coefficient are estimated for both natural and forced convection modes under steady state condition, by varying the heat input from 4 W to 24 W, and the air velocity from 1 m/s to 4 m/s. The coolant velocity and heat input to achieve minimum R_t are found out and the corresponding effective thermal conductivity is calculated. The transient temperature distribution in the finned heat pipe is also observed. The experimental observations are verified by simulation using ANSYS 10. The results show that the air velocity, power input and heat pipe orientation have significant effects on the performance of finned heat pipes. As the heat input and air velocity increase, total thermal resistance decreases. The lowest value of the total thermal resistance obtained is 0.181 °C/W when heat input is 24 W and air velocity 3 m/s. The experimental and simulation results are found in good agreement.     

The effect of different inlet geometries on laminar flow combined convection heat transfer inside a horizontal circular pipe

The effect of different inlet geometries on laminar air flow combined convection heat transfer inside a horizontal circular pipe has been experimentally investigated for Reynolds number range of 400–1600, and the Grashof number range from 3.12 × 10⁵ to 1.72 × 10⁶. The experimental setup consists of an aluminum circular pipe as a heated section with 30 mm inside diameter and 900 mm heated length (L/D = 30) with different inlet geometries. A wall boundary heating condition of a uniform heat flux was imposed. The inlet configurations used in this paper are calming sections having the same inside diameter as the heated pipe but with variable lengths of L_calm. = 600 mm (L/D = 20), L_calm. = 1200 mm (L/D = 40), L_calm. = 1800 mm (L/D = 60), L_calm. = 2400 mm (L/D = 80), sharp-edged and bell-mouth. It was found that the surface temperature values for calming section length corresponding to (L/D = 80) were higher than other inlet geometries due to the lower mass flow rate and higher flow resistance. It was also observed that the Nusselt number values for bell-mouth inlet geometry were higher than other inlet geometries due to the differences in the average temperatures and densities of the air. The average heat transfer results were correlated with an empirical correlation in terms of dependent parameters of Grashof, Prandtl and Reynolds numbers. The proposed correlation was compared with available literature and it shows reasonable agreement.     

Investigation of heat transfer and pressure drop of CO2 two-phase flow in a horizontal minichannel

An innovative cooling system based on evaporative CO₂ two-phase flow is under investigation for the tracker detectors upgrade at CERN (European Organization for Nuclear Research). The radiation hardness and the excellent thermodynamic properties emphasize carbon dioxide as a cooling agent in the foreseen minichannels. A circular stainless steel tube in horizontal orientation with an inner diameter of 1.42 mm and a length of 0.3 m has been used as a test section to perform the step-wise scanning of the vapor quality in the entire two-phase region. To characterize the heat transfer and the pressure drop depending on the vapor quality in the tube, measurements have been performed by varying the mass flux from 300 to 600 kg/m² s, the heat flux from 7.5 to 29.8 kW/m² and the saturation temperature from ?40 to 0 °C (reduced pressures from 0.136 to 0.472). Heat transfer coefficients between 4 kW/m² K and 28 kW/m² K and pressure gradients up to 75 kPa/m were registered. The measured data was analyzed corresponding to the dependencies on heat flux, mass flux and saturation temperature. A database has been established containing about 2000 measurement points. The experimental data was compared with common models recently developed by Cheng et al. [1], [2] to cross check their applicability. The overall trends and experimental data were reproduced as predicted by the models before the dryout onset, and deviations have been analyzed. A modified friction factor for the pressure drop model [1] in mist flow has been proposed based on the experimental data.     

Utilization of cool ceiling with roof solar chimney in Thailand: The experimental and numerical analysis

The objective of this paper is to study the benefits of application of solar chimney on the south roof and cool metal ceiling on the north roof through the experiment in a detached building called a controlled cell, and the related numerical model constructed from a computational fluid dynamics (CFD) program. The experimental results are used for calculation of values of heat transfer coefficient of the cool ceiling and evaluation of the mean cooling potential of the combined passive cooling system. The two-dimensional numerical models generated by the CFD program use the mean values of wall temperatures in the application of solar chimney in the controlled cell as the boundary conditions. The effects of cool ceiling on the temperature, velocity and airflow rate in the controlled cell are investigated through the numerical model in which the north ceiling temperature is reduced by 2–4 °C from the measured value of 32.8 °C. The mean cooling potential of the application of combined system is found to be two times higher than the application of the solar chimney. Good agreements between the predicted and experimental results are obtained from the comparison of temperature and volume flow rate at the middle section of the controlled cell. The reduction of north ceiling temperature in the free-convection numerical model shows the decrease of air temperature in the upper region of the room by 0.5–0.7 °C from the original value of 33.3 °C, and the increase of volume flow rate by 12%.     

Effect of functionalized MWCNTs/water nanofluids on thermal resistance and pressure fluctuation characteristics in oscillating heat pipe

An influence of multi-walled carbon nanotube (MWCNT) based aqueous nanofluids with different concentrations on the heat transport and the relevant pressure distribution in oscillating heat pipe (OHP) has been investigated. The present paper describes the heat transfer phenomena in terms of thermal resistance, pressure and frequency of pressure fluctuation in multi-loop oscillating heat pipe (OHP) charged by aqueous nanofluids with MWCNT loadings of 0.05 wt.%, 0.1 wt.%, 0.2 wt.% and 0.3 wt.%. The multi-loop OHP with 3 mm inner diameter has been conducted in the experiment at 60% filling ratio. Experimental results show that thermal characteristics are significantly inter-related with pressure distribution and strongly depend upon the number of pressure fluctuations with time. The investigation shows that the 0.2 wt.% MWCNTs based aqueous nanofluids obtain maximum number of the fluctuation frequency and low thermal resistance at any evaporator power input. Based on the experimental results, we discuss the reasons for enhancement and decrement of thermal characteristics of the nanofluids.     

Unsteady fluid mechanics and heat transfer study in a double-tube air–combustor heat exchanger with porous medium

Fluid mechanics and heat transfer are studied in a double-tube heat exchanger that uses the combustion gases from natural gas in a porous medium located in a cylindrical tube to warm up air that flows through a cylindrical annular space. The mathematical model is constructed based on the equations of continuity, linear momentum, energy and chemical species. Unsteady fluid mechanics and heat transfer by forced gas convection in the porous media, with combustion in the inner tube, coupled to the forced convection of air in the annular cylindrical space are predicted by use of finite volumes method. Numerical simulations are made for four values of the annular air flow Reynolds number in the range 100 ? Re ? 2000, keeping constant the excess air ψ = 4.88, the porosity ε = 0.4, and the air–fuel mixture inlet speed Uo = 0.43 m/s. The results obtained allow the characterization of the velocity and temperature distributions in the inner tube and in the annular space, and at the same time to describe the displacement of the moving combustion zone and the annular porous media heat exchanger thermal efficiency. It is concluded that the temperature increase is directly related to the outer Reynolds number.     

A new type adsorber for adsorption ice maker on fishing boats

A new type of adsorber for an adsorption ice maker on fishing boats, which uses a compound adsorbent (activated carbon and CaCl₂) and ammonia working pair, is designed. This type of heat pipe adsorber solves the problem of incompatibility between ammonia, copper, seawater and steel. The heating/cooling power for the adsorption/desorption process of the adsorbent, which is required to be transferred by one heat pipe in the adsorber, is computed by the test results of the adsorbent, and the heat transfer performance of one heat pipe in the adsorber is simulated according to the theory of the two phase closed thermosyphon. The heat transfer performance of the heat pipe can meet the heat demands for adsorption/desorption of the adsorbent when the evaporating temperature is −15 °C and the cycle time is 10 min. A test unit is set up to test the heating/cooling performance of the heat pipe type adsorber, and the experimental results are coincident with the simulation. The performance of a two bed adsorption ice maker with heat pipe adsorbers is predicted, and the cooling power is about 17.1–17.8 kW at the evaporating temperature of −15 °C and cycle time of 10 min with mass recovery between two 29 kg compound adsorbent beds.