Heat transfer characteristics in a small-scale fluidized bed boiler

Heat transfer characteristics in a small-scale fluidized bed boiler (2MW_th) were studied using lignite and corn cob as fuels. Depending on air velocity, the heat transfer rates from bed to water membrane wall and from hot flue gas to convective tube bank were in the ranges 75–55% and 25–45% of the total heat absorbed by the boiler, respectively. At designed capacity, the heat transfer flux based on bed cross sectional area and on water membrane wall area were about 0·45 and 0·15 MWm⁻², respectively. Under the conditions studied, it was found that the overall heat transfer coefficient between bed and water membrane wall was 100–300 W m⁻² K⁻¹, whereas that between flue gas and convective tube bank was 10–30 Wm⁻² K⁻¹. The study of heat transfer to a horizontal tube immersed in the bed as well as placed in the freeboard region were also studied. The effective heat transfer coefficients were found to be 300–800 W m⁻² K⁻¹ for in-bed tube and 30–150 W m⁻² K⁻¹ for the freeboard region, depending on air velocity. Comparison of these data with those predicted by both modelling and correlation reported in the literature was also made. For the immersed tube, good agreement was observed for low air velocity, while at high air velocity the experiment produced results twice those estimated from modelling and correlation. For the freeboard region, the model gave a fair prediction.     

Effect of lubricating oil contamination on evaporation in refrigerants R12 and R22

An investigation has been made into the effect of oil concentration on evaporation heat transfer coefficients in refrigerant-oil mixtures flowing in a horizontal tube. A new correlation is presented for heat transfer coefficients in convective evaporation of refrigerant-oil mixtures that predicts the results of the present study within approximately ±20%. The paper reports measurements of evaporation heat transfer coefficients in refrigerants R12 and R22, both oil-free and with two concentrations of Shell Clavus 32 oil. A 1·8 m long ⅜ in O/D copper tube (8·05 mm I/D) was used, at evaporation temperatures of −5°C, 0°C and +5°C. Heat flux and mixture mass velocity were kept constant at 2500 W m⁻² and 155 kg m⁻² s⁻¹, respectively, and measured coefficients were in the range of 1400 to 3900 W m⁻² K⁻¹. The results showed that, for a complete evaporator, 2% oil may be expected to increase the heat transfer coefficient by 12%, but 10% oil returns the coefficient to oil-free values.     

A study of heat transfer in a circulating fluidized bed

An experimental investigation was carried out to study the effects of operating parameters on the local bed-to-wall heat transfer coefficient in a 4.5 m tall, 0.150 m diameter circulating fluidized bed with a bed temperature in the range of 65°C to 80°C, riser flow rate varying from 1400 litres/min to 2000 litres/min, bed inventory in the range of 15 kg to 25 kg of sand, and average sand sizes of 200 μm, 400 μm and 500 μm. A heat flux probe was attached to the riser wall at five different vertical locations for measuring the heat flux from the bed to the wall surface. From the present work, the heat transfer coefficient in the dilute phase was found to be in the range of 62 to 83 W/m²K, 51 to 74 W/m²K, and 50 to 59 W/m² K for sand sizes of 200 μm, 400 μm and 500 μm, respectively. Relevant mathematical correlations were developed to predict local heat transfer coefficient based on the results of the practical work.     

Estimation of bed expansions in a freely-bubbling three-dimensional gas-fluidized bed

Knowledge of bed expansions is important in the design and operation of gas–solid fluidized beds. This paper presents a study on the estimation of expanded bed height in a large three-dimensional gas-fluidized bed with a square section of 0·61×0·61 m². All experiments were performed at the freely bubbling mode and the bed expansions were recorded by a video camera. Bed materials were used 593 μm raw perlite and 1233 μm sand falling within the categories of Geldart's Groups B and D, respectively. The bed height at minimum fluidization ranged from 0·0398 to 0·3176 m, while the excess air velocity from 0·034 m s⁻¹to 0·7453 m s⁻¹. Equations related to the bed expansion were given using a modified form of two-phase theory of fluidization. A correlation for the average bed expansion (void fraction) was also presented that has been derived from the principal form found successful in gas–liquid systems as follows: R=0·5482 d^−0·129_p(U_o−U_mf)^0·111 with an average deviation of less than 1%. The experimental findings were compared with previously reported results and were discussed in the light of available correlations. © 1998 John Wiley & Sons, Ltd.     

Study of heat transfer between an over-bed oil burner flame and a fluidized bed during start-up: Determination of the flame to bed convection coefficient

A study of the heat transfer processes between an over-bed burner flame and a fluidized bed during start-up as been conducted. Owing to the difficulty of estimating the flame to bed convection coefficient in an industrial boiler, convection coefficients were determined using a laboratory bench scale unit. Such convection heat transfer coefficients are obtained for 3 kg, 4 kg and 5.5 kg initial bed inventories by combining measured temperatures and flow rates with a mathematical model representing the complex energy exchange in the system. Results show that the height of the fluidized bed and its distance to the flame are an important factor in the overall heat transfer process, both by convection and radiation. For 5.5 kg, 4 kg and 3 kg initial bed inventories, the convection coefficients obtained, at the end of start-up, are 180 ± 30 W/m² K, 150 ± 20 W/m² K and 95 ± 10 W/m² K respectively. The determined convection coefficients can be utilized in the future as guides in the design of start-up systems for BFB boilers. The energy analysis performed also identified the major sources of heat losses in the bubbling fluidized bed.     

Effects of particle sizes on methanol steam reforming for hydrogen production in a reactor heated by waste heat

This paper reports the effects of particle sizes on methanol steam reforming for hydrogen production in a reactor heated by waste heat. The unsteady model was set up, which has been applied to investigate the effects of particle sizes (1.77 mm–14.60 mm) on particle temperature, heat transfer quantity, overall coefficient of heat-transfer, etc. The heat transfer performance of waste heat recovery heat exchanger is improved when the particle size increases, which is conducive to increase hydrogen production. The particle temperature change rate, the specific enthalpy change rate, the moving velocity of the maximum heat release rate particle, the contribution rate of solid phases, the heat release rate and the overall coefficient of heat-transfer increase, but the effective time of heat transfer decreases. When the particle size increases from 1.77 mm to 14.60 mm, the solid phase average contribution rate increases from 89.43% to 94.03%, the overall coefficient of heat-transfer increases from 1.39 W m⁻² K⁻¹ to 13.41 W m⁻² K⁻¹, the heat release rate increases from 48.9% to 99.9% and the effective time of heat transfer reduces from 48 h to 6.7 h.     

Dropwise Condensation Heat Transfer on Plasma-Ion-Implanted Small Horizontal Tube Bundles

Stable dropwise condensation of saturated steam was achieved on stainless-steel tube bundles implanted with nitrogen ions by plasma ion implantation. For the investigation of the condensation heat transfer enhancement by plasma ion implantation, a condenser was constructed in order to measure the heat flow and the overall heat transfer coefficient for the condensation of steam on the outside surface of tube bundles. For a horizontal tube bundle of nine tubes implanted with a nitrogen ion dose of 10¹⁶ cm^{? 2}, the enhancement ratio, which represents the ratio of the overall heat transfer coefficient of the implanted tube bundle to that of the unimplanted one, was found to be 1.12 for a cooling-water Reynolds number of about 21,000. The heat flow and the overall heat transfer coefficient were increased by increasing the steam pressure. The maximum overall heat transfer coefficient of 2.22 kW · m^?2· K^?1 was measured at a steam pressure of 2 bar and a cooling-water Reynolds number of about 2,000. At these conditions, more dropwise condensation was formed on the upper tube rows, while the lowest row received more condensate, which converted the condensation form to filmwise condensation.     

Expansion of multi-jet bed with large particles

An experimental investigation under cold conditions was made to study the effect of some operating parameters on average porosity in a 1·1 m long, 0·35 m wide and 1·2 m high multi-jet bed (Ingnifluid type) with air flow rate varying from 1200 m³ h⁻¹ to 3500 m³ h⁻¹ and total bed inventory from 26 kg to 45 kg. Rice, peas and one rice-pea mixture (mass ratio 70–30) of sizes 1·95 mm, 5.0 mm and 2·44 mm, respectively, were used as bed material to simulate coal particles. Average bed porosity was estimated based on pressure drop along the bed height. It was found to be in the range 0·58 to 0·72, 0·51 to 0·62 and 0·55 to 0·65 for rice, peas and rice-pea mixture, respectively. One mathematical correlation has been developed from the experimental results to predict average porosity as a function of air flow rate, total bed inventory and particle size used. This correlation is developed for hydrodynamic modelling of an industrial multi-jet combustor.     

Axial and radial heat transfer studies in a circulating fluidized bed

The axial and radial variation of the heat transfer coefficient in a circulating fluidized bed riser column, and the effect of operating parameters thereon, are investigated. The experimental set-up consists of a riser column of 102 mm×102 mm in bed cross-section, 5·25 m in height with a return leg of the same dimensions. The unit is fabricated with plexiglass columns of 0·6 m in length which are interchangeable with one another. Two axial heat transfer test sections of 102 mm×102 mm in cross-section, 500 mm in height, and made of mild steel, are employed for the axial heat transfer study and one horizontal tube section of 22·5 mm OD made of mild steel is employed for the radial heat transfer study. The primary air velocity is varied between 4·21 and 7·30 m s⁻¹. Local sand of mean size (d_p) 248 μm is used as the bed material. One empirical model with the help of dimensional analysis has been proposed to predict the heat transfer coefficient to a bare horizontal tube in a CFB riser column and the model results are validated with the experimental data; good agreement has been observed. © 1997 John Wiley & Sons, Ltd.     

Modelling airflow,heat transfer and moisture transport around a standing human body by computational fluid dynamics

In this study a combined computational model of a room with virtual thermal manikin with real dimensions and physiological shape was used to determine heat and mass transfer between human body and environment. Three dimensional fluid flow, temperature and moisture distribution, heat transfer (sensible and latent) between human body and ambient, radiation and convection heat transfer rates on human body surfaces, local and average convection coefficients and skin temperatures were calculated. The radiative heat transfer coefficient predicted for the whole-body was 4.6 W m^− 2 K^− 1, closely matching the generally accepted whole-body value of 4.7 W m^− 2 K^− 1. Similarly, the whole-body natural convection coefficient for the manikin fell within the mid-range of previously published values at 3.8 W m^− 2 K^− 1. Results of calculations were in agreement with available experimental and theoretical data in literature.     

Effects of ellipsoidal and regular hexahedral particles on the performance of the waste heat recovery equipment in a methanol reforming hydrogen production system

Hydrogen production from methanol has attracted attention due to its wide range of raw material sources and mature technology. Using waste heat of industrial high temperature solid particles like blast slag and steel slag etc. To provide vaporization heat and reaction heat for the reaction between methanol and water is an emerging technology for hydrogen production from methanol, which can save additional thermal energy resources. Herein, the performances of equipment that uses the waste heat of ellipsoidal and regular hexahedral particles to provide a heat source for methanol to hydrogen were explored by the DEM-CFD method. Compared with spherical particles of the same equivalent diameter, ellipsoidal and regular hexahedral particles have poor fluidity in the stagnant area, and the empty area is enlarged and irregular in shape. The average velocity peaks of the ellipsoidal and regular hexahedron particles are larger than those of spherical particles, and the overall mean velocity fluctuation of ellipsoidal particles is similar to that of spherical particles while the regular hexahedron particles' is larger. The average temperature drop rate of the ellipsoidal and regular hexahedral particles is slower than that of spherical particles, the uniformity of temperature distribution is worse than that of spherical particles. The ellipsoidal and regular hexahedral particles’ average effective heat transfer coefficient is smaller than that of spherical particles, and the heat transfer effect is weaker than that of spherical particles. The effective heat transfer coefficient of ellipsoidal particles is 2.95 W/(m⁻²∙K⁻¹) lower than that of spherical particles and the effective heat transfer coefficient of hexahedral particles is 6.09 W/(m⁻²∙K⁻¹) lower than that of spherical particles. Therefore, compared with the spherical particles of the same equivalent diameter, ellipsoidal and regular hexahedral particles produce less hydrogen.     

Condensation heat transfer coefficients of the zeotropic refrigerant mixture R-22/R-142b in smooth horizonal tubes

Heat transfer coefficients during condensation of the zeotropic refrigerant mixture R-22 with R-142b are presented. Measurements were obtained at different mass fractions in a smooth horizontal tube. All measurements were conducted at a high condensing saturation pressure of 2.43 MPa, which corresponds to a condensation temperature of 60 °C for R-22. The measurements were taken in 8.11 mm inner diameter smooth tubes with lengths of 1 603 mm. The heat transfer coefficients were determined with the Log Mean Temperature Difference equations. It was found that at low mass fluxes, between 40 kg·m⁻²·s⁻¹ to 350 kg·m⁻²·s⁻¹, the refrigerant mass fraction influences the heat transfer coefficient by up to a factor of two. The heat transfer coefficients decrease as the fraction of R-142b is increased. At high mass fluxes, of 350 kg·m⁻²·s⁻¹ and more the heat transfer coefficients were not strongly influenced by the refrigerant mass fraction. The average heat transfer coefficient decreased by only 7% as the refrigerant mass fraction changed from 100% R-22 to 50%/50% R-22/R142b.     

Waste heat utilization in a ceramic industry

The scope for energy conservation in a ceramic factory was studied. It was found that the sensible heat of the flue gas from kilns could be used to heat the casting shop. A pebble bed waste heat regenerator was chosen to collect the sensible heat of the flue gas. Experiments were performed on a pilot plant scale pebble bed to study its characteristics using a flue gas stream whose temperature and composition were similar to the factory conditions. The results of the heating cycle showed that 14 500 kcal h⁻¹ of heat can be collected from flue gas in a 0·126 m³ bed. This information was used to design a complete retrofit system. Financial analysis of the proposed project showed that it has a pay-back period of 15 months. © 1997 by John Wiley & Sons, Ltd.     

Measurement of the thermal properties of TiFe0.85Mn0.15 and its hydrides

A high vacuum/high pressure measuring apparatus for the study of equilibrium thermophysical properties, heat transfer parameters and dynamic reaction thermal response of hydrogen storage alloys has been constructed. The absolute thermal conductivity of massive hydridable TiFe_0.85Mn_0.15 alloy has been determined, as well as the effective thermal conductivities of identical particulated material in both unhydrided and hydrided form using the steady-state plate method.The effective conductivity of powders with particle sizes between 0.075 and 0.425 mm is found to be strongly temperature- and pressure-dependent, with values ranging from 0.1 W m⁻¹ K⁻¹ in high vacuum of 10⁻³ Pa, to 1.5 W m⁻¹ K⁻¹ at high He or H₂ pressures up to 5.5 MPa. Hydriding of the powders does not significantly change the effective conductivity, whereas packing density, contact pressure and particle size does so. Massive TiFe_0.85Mn_0.15 has a surprisingly high absolute thermal conductivity of 77.2 W m⁻¹ K⁻¹, which is temperature independent in the range from 0–150°C.     

Condensation studies using cross-corrugated polymer film compact heat exchanger

Heat transfer has been examined in a polymer film compact heat exchanger between cross flowing liquid and gas. Condensation of water vapour through a non-condensable gas was used to supply heat through a corrugated poly-ether-ether-ketone (PEEK) film to a cooling liquid. Measurements of heat transfer rates in the system indicated overall heat transfer coefficients in the range of 50–300 W m⁻² K⁻¹ were achieved. Visual analysis and pressure drop measurements were used to provide insight into the fluid flow and the models used for heat transfer.     

Influence of packed honeycomb ceramic on heat extraction rate of packed bed embedded heat exchanger and heat transfer modes in heat transfer process

Under the condition that the transient oxidation heat extraction process of coal mine ventilation air methane (VAM) is equivalent to a series of steady state process, the steady state heat extraction experiment platform is built. The influence of the honeycomb ceramic packed in heat extraction zone and its two-side space on heat extraction rate and heat transfer modes is investigated. The experimental results show that the honeycomb ceramic packed in heat extraction zone two-side space can always strengthen heat extract ion of heat exchanger by increasing gas physical flow velocity in bed and radiation heat exchanging area and disturbing heat exchanger leeward side flow field. The contradictory dual characteristic of the influence of the honeycomb ceramic packed in heat extraction zone on heat exchanger heat extraction rate determines that the honeycomb ceramic has no great influence on heat extraction rate and doesn't always strengthen heat exchanger heat extraction. Contribution of heat transfer modes on packed bed embedded heat exchanger heat extraction is investigated using the method of coating heat exchanger outer surface silver; the experimental result shows that 55% contribution of packed bed embedded heat exchanger heat extraction rate is from radiation when gas mass flow rate is 0.15 kg·s^− 1·m^− 2 and its temperature is 1113 k; with the gas temperature being increased further, radiation will become the main way of packed bed embedded heat exchanger heat extraction.     

Absorption of water vapour in the falling film of water–lithium bromide inside a vertical tube at air-cooling thermal conditions

In the absorbers of air-cooled water–lithium bromide absorption chillers, the absorption process usually takes place inside vertical tubes with external fins. In this paper we have carried out an experimental study of the absorption of water vapour over a wavy laminar falling film of water–lithium bromide on the inner wall of a smooth vertical tube. The control variables for the experimental study were; absorber pressure, solution mass flow rate, solution concentration and cooling water temperature. Relatively high cooling water temperatures were selected to simulate air-cooling thermal conditions. The parameters considered to assess the performance of the absorber were; the mass absorption flux, the outlet solution degree of subcooling and the falling film heat transfer coefficient. The results indicate that in water cooling thermal conditions the mass absorption fluxes are in the range 0.001–0.0015 kg·m⁻²·s⁻¹, whereas in air-cooling thermal conditions the range of mass absorption values decreases to 0.00030–0.00075 kg·m⁻²·s⁻¹.     

Heat transfer in a crater bed

The aim of this work is to study heat transfer in a laboratory scale crater bed, which was set up from a cylindrical acrylic/quartz tube, using sand as the bed particle. The bed employs a downward gas jet from a nozzle which causes the particles to ascend fountain-like into the freebroad, leaving a crater on the bed surface. After reaching a certain height, these particles will descend again to the bed surface and move into the crater, where the cycle or circulation pattern starts again. The study had been separated into three parts. Firstly, the void fraction of the bed fountain zone was studied by direct measurement of the ascending sand weight within the specific volume. Secondly, the convection heat transfer coefficients between the fountain zone and the external surface of the gas inlet tube were determined by measuring the quantity of heat loss from an electrical heater that was wrapped on the outside surface at desired positions of the gas inlet tube. Thirdly, the radiation heat transfer coefficients were evaluated by heat balance of LPG combustion in the crater bed. From experimental results, the void fraction of the fountain zone could be approximated as a dilute bed (>0.98). For convective heat transfer coefficients, the value found experimentally varied from 80–260 W/m² K depending on the experimental conditions, showing an increase when the gas velocity increases, and a decrease along the height of the gas inlet tube. Radiation heat transfer coefficients, the values of which are (within the experimental temperature range), the same order as the convective mode, increase when the bed temperature is increased and when the bed particle diameter is decreased. Empirical correlations for both bed voidage and heat transfer coefficients are proposed. The combined model, gas and particle convection and the published data on radiation heat transfer, showed good prediction when compared with experimental data.     

Investigation of the bubble behaviour at the free surface of a large three-dimensional gas fluidized bed

Gas fluidization is generally associated with the formation of bubbles that critically influence the performance of fluidized bed processes (FBPs). Therefore, in the design, simulation and operation of FBPs, it is very essential to know the behaviour of the bubbles at the free surface. The size and growth of bubbles play an important role for determining properties such as bed expansion, solids entraiment, in-bed heat transfer and solid mixing. This paper presents a study on the behaviour of bubbles at the free surface of a large three dimensional gas-fluidized bed with square section of 61×61 cm². Measurements were carried out to determine the effects of bed height and excess air velocity on the bubble eruption diameter, frequency and bubble fraction. All experiments were performed at freely bubbling mode and the flow characteristics of bubbles were recorded by a video camera. Bed materials used were 593 μm raw perlite and 1233 μm sand falling within the categories of Geldarts Groups B and D, respectively. The fixed bed height ranged from about 8–18 cm for raw perlite and 9–26 cm for sand. The excess air velocity was varied between 0·5 and 1·75 cm s⁻¹ for raw perlite and 13 and 25 cm s⁻¹ for sand. Equations related to the bubble count, frequency, flow area shape factor and through-flow coefficient were given using a modified form of two-phase theory of fluidisation. Observations were made to validate the two-phase theory for two different particles. The flow area shape factor was in the range of 0·47–0·81 for raw perlite and 0·20 to 0·57 for sand, with mean values of 0·6 and 0·4, respectively. The through-flow coefficient was found to be between −0·68 and 2·82 for raw perlite and between 3·27 to 15·87 for sand, and was larger than predicted values of classical bubble models. © 1998 John Wiley & Sons, Ltd.     

Suspension-to-wall heat transfer in a 12-MWth circulating fluidized-bed combustor

An experimental investigation was carried out to study the effects of operating parameters on the local suspension-to-wall heat transfer in the combustor of a 12-MW_th circulating fluidized-bed (CFB) boiler. The heat transfer coefficients were measured with a conduction-type heat flux meter at five different vertical levels. The measurements covered a range of superficial gas velocities from 4 to 6 m/s, a bulk bed temperature from 800 to 850 °C and a suspension density from 6 to 70 kg/m³ for 270-μm silica sand particles. The heat transfer coefficient for the membrane wall in the combustion chamber of the CFB boiler was in the range of 100 to 180 W/m² K for the range of operating conditions employed in this work. The heat transfer coefficient decreased with increasing height and increased with increasing bulk bed temperature, superficial gas velocity and suspension density. Based on the experimental data, a simple correlation is proposed for predicting the suspension-to-membrane wall heat transfer coefficient. The results were analysed and compared with the experimental data of other workers.