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Steel industry is high energy-consuming industry, and its waste heat recovery is critical ly important for energy utilization. In this study, pipeline bundle is used to enhance heat transfer in waste heat recovery device, and associated gas-solid heat transfer and energy utilization performance with different pipeline arrangement, pipe diameter and shape of internal component are further analyzed. The temperatures of gas and particle in device with pipeline bundle periodically fluctuate in horizontal direction, and those in staggered system distribute more uniformly than those in paralleled system. Compared with paralleled device, exergy and waste heat utilization efficiency of staggered device have been improved, and they are both higher tha n those without pipeline. As pipe diameter increases, e xergy and waste heat utilization efficiency first increases and then decreases, and they reach the maxima with optimal pipe diameter. As the width of internal component keeps constant, influence of its shape on heat transfer is very little.

The steel industry has always been the top priority of national industry, and energy consumption of blast furnace steelmaking can reach 40% of total energy consumption in steel industry, 10% - 15% of total energy consumption in the country, belonging to high energy-consuming industry [

Shigaki et al. [

At present, there are few studies on the influence of waste heat recovery device with different internal components on gas-solid heat transfer and waste heat utilization. In this paper, gas-solid heat transfer and waste heat utilization efficiency in waste heat recovery device with different pipeline arrangements, pipe diameter and shape of internal component is simulated. In addition, effects of inlet gas velocity, inlet gas temperature, particle diameter and particle porosity are further considered.

The schematic diagram of waste heat recovery device is shown in

1) The simulation process is steady state, and operating parameters are constant.

2) Porous media model is used for simulation. Porous media is homogenous material, and the temperature of the particles is uniform.

3) Considering convection and heat conduction between particle and fluid in porous media, regardless of influence of radiation and heat loss.

Continuous equation:

∂ ρ u i ∂ x i = 0 (1)

Momentum equation:

∂ ∂ x i ( ρ u i u k ) = ∂ ∂ x i ( μ ∂ u k ∂ x i ) − ∂ p ∂ x i + ρ g + S i (2)

where ρ is density, kg/m^{3}; u_{i} is flow velocity in i direction, m/s; P is pressure, Pa; g is acceleration of gravity, m/s^{2}; S_{i} is momentum source term; μ is viscosity, Pa·s.

When the porous medium model is used, the momentum source term is added to the equation to add the effect of porous medium on the fluid.

S i = − ( μ α u i + 1 2 C 2 ρ g | u | u i ) (3)

The viscous resistance coefficient and the inertia resistance coefficient can be expressed as [

1 α = 150 ( 1 − ε ) 2 ε 3 d p 2 (4)

C 2 = 3.5 ( 1 − ε ) ε 3 d p (5)

where ε is particle porosity; d_{s} is particle diameter, m.

The local thermal non-equilibrium:

∂ ∂ x i ( ρ s c p , s u i T s ) = ∂ ∂ x i ( ( 1 − ε ) λ s ∂ T ∂ x i ) − h e α ( T s − T g ) (6)

∂ ∂ x i ( ρ g c p , g u i T s ) = ∂ ∂ x i ( ε λ g ∂ T ∂ x i ) + h e α ( T s − T g ) (7)

where T is temperature, K; λ is thermal conductivity, W/(m·K); h and h_{e} are heat transfer coefficient and effective heat transfer coefficient, W/(m^{2}·K); c_{p} is specific heat of particle, J/(kg·K); subscripts “s” and “g” represent particle and gas respectively.

According to the Achenbach, the specific surface area can be expressed as [

α = 6 ( 1 − ε ) d p (8)

Also consider the effect of the internal heat transfer of the particles on the overall heat transfer coefficient, the heat transfer coefficient is corrected. The effective heat transfer coefficient according to Jefferson can be expressed as [

1 h e = 1 h ( 1 + B i 5 ) β 2 (9)

where Nu is used the association proposed by Ranz [

N u = h d p k g = 2.0 + 0.6 R e 1 / 2 P r 1 / 3 (10)

The quantity of waste heat utilization is expressed as:

Q g = q g , i n ( c p , o u t T g , o u t − c p , i n T g , i n ) (11)

The exergy of gas (Ex) is expressed as:

E x = Q g ⋅ ( 1 − T 0 T g , o u t − T g , i n ln T g , o u t T g , i n ) (12)

The waste heat utilization efficiency (η) is expressed as:

η = ∫ T s , o u t T s , i n ρ s c p , s d T ∫ T 0 T s , i n ρ s c p , s d T (13)

where β is heat capacity ratio; T_{0} is ambient temperature, K; q is air mass flow rate, kg·s^{−1}; subscripts “in” and “out” represent the inlet and outlet.

The equations are solved by Fluent. The local thermal non-equilibrium energy equations are applied by UDF and UDS. The material density, specific heat and other parameters are set by UDF and Fluent database, grid is divided by ICEM, and pressure velocity coupling is solved by SIMPLE algorithm. The turbulence model adopts k-ε model, and residual convergence criterion is less than 10^{−6}.

The gas density is 1.225 kg/m^{3}, air specific heat can be calculated by Equation (14); particle gas density is 2900 kg/m^{3}, particle specific heat can be calculated by Equation (15).

c p , g = 1908.911 + 7.054 × 10 − 1 T g + 1.67 × 10 − 3 T g 2 − 1.225 × 10 − 6 T g 3 + 3.080 × 10 − 10 T g 4 (14)

c p , s = 1014 + 6.21 × 10 − 2 T g − 0.347 × 10 8 T g − 2 (15)

This paper uses experiment of Feng [

Temperature and velocity evolution in horizontal direction of paralleled device are shown in

Temperature and velocity evolution in horizontal directions of staggered device are shown in

Experimental condition | Outlet particle temperature (K) | Outlet gas temperature (K) | ||||
---|---|---|---|---|---|---|

Measured | Calculated | Relative error | Measured | Calculated | Relative error | |

condition 1 | 394 | 412 | 3.74% | 795 | 784 | −2.24% |

condition 2 | 379 | 398 | 3.62% | 748 | 735 | −2.94% |

condition 3 | 383 | 396 | 2.56% | 756 | 744 | −2.66% |

velocity in different devices. As inlet gas velocity increases, outlet gas temperature and outlet particle temperature drops. The increase of inlet gas velocity results in an increase in heat transfer coefficient, which leads to a decrease in outlet particle temperature. At the same time, larger mass flow rate of gas requires more energy, resulting in a decrease in outlet gas temperature.

increases and outlet particle temperature decreases, because the reduction of particle size will increase effective heat transfer coefficient between gas and particle.

exergy decreases, and the waste heat utilization efficiency increases. It is shown that although particle porosity decrease can increase the exergy, it will also reduce the waste heat utilization efficiency. It is necessary to choose a reasonable value to use.

Form

_{g,in} = 1.2 m/s. As pipe diameter increases, exergy and waste heat utilization efficiency first increases and then decreases. As pipe diameter is 0.15 m, exergy and waste heat utilization efficiency reach the maxima of 4.26 GJ/h and 82.5%.

heat utilization efficiency also keeps almost constant.

The heat transfer and energy utilization performance in waste heat recovery device with different pipeline arrangements, pipe diameter and shape of internal component is simulated and studied, and conclusions are as follows.

1) In waste heat recovery device with pipeline bundle, gas velocity and temperature in horizontal direction periodically change. Compared with paralleled arrangement, the system with staggered arrangement has more uniform temperature distribution and higher outlet gas temperature.

2) By using pipeline bundle, gas-solid heat transfer and energy utilization can be significantly enhanced. The exergy and waste heat utilization efficiency of staggered pipeline system is higher than that in paralleled one, and that without pipeline is lowest.

3) As pipe diameter increases, exergy and waste heat utilization efficiency first increases and then decreases, and they reach the maxima with optimal pipe diameter. The shape of internal component with identical width has little effect on exergy and waste heat utilization efficiency.

4) As inlet gas temperature or particle diameter increases, exergy and waste heat utilization efficiency decreases; as particle porosity increases, exergy decreases, and waste heat utilization efficiency increases; as inlet gas velocity increases, exergy increases first and then decreases, and waste heat utilization efficiency increases all the time.

This paper is supported by Natural Science Foundation of Guangdong Province (2017B030308004) and National Natural Science Foundation of China (U1601215, 51961165101).

The authors declare no conflicts of interest regarding the publication of this paper.

Tang, E.M., Ding, J. and Lu, J.F. (2020) Heat Transfer and Energy Utilization of Waste Heat Recovery Device with Different Internal Component. Energy and Power Engineering, 12, 88-100. https://doi.org/10.4236/epe.2020.122007