双组元25N推力器脉冲工作模式下的传热特性分析及改进

在离心式双组元25N推力器的脉冲工况温度稳定性试验中,有16%的工况均发生了推力器稳态温度跃迁导致的喷注器头部高温过热的问题。本研究针对该问题开展了深入的机理分析,并通过优化推力器局部传热特性以消除其对于特殊脉冲工况的应用限制和风险。通过液滴撞壁试验,分析了不同条件下推进剂液膜发展的规律和机理:一旦壁面温度达到了莱登弗斯特温度以后,液滴与壁面的换热能力将大幅下降,严重影响液膜冷却的效率。通过Amesim软件进行的传热仿真进一步表明,一旦因为热回浸现象导致雾化液滴与涂层壁面的作用机制发生变化,进而影响冷却液膜的形成,就会使推力器的温度分布从正常的第一稳态进入无液膜的第二高温稳态。仿真结果基本复现了热稳定性试验中出现的故障模式。针对该故障机理改进了喷注器的结构导热性,将喷注器的花篮连接结构的导热面积增加了1.5倍,可以有效分担液膜的热流负担,抑制液膜进入莱登弗斯特态,使得推力器的脉冲工作可靠性大幅提高,令温度稳定性试验的通过率提升至100%。

Experimental study of low-temperature electrical radiant floor heating system

To evaluate the thermal performance of a low-temperature electrical radiant floor heating system,an experimental facility equipped with a constant temperature chamber and different specimen floors is designed and built.The heating cable is installed in the floor slab with a unit-rated power of 30 W/m.Twenty-four different schemes are worked out and tested,which include three kinds of composite floor structures and eight kinds of cable distances.The cable distances are 30,40,50,60,80,100,130,150 mm.The main affective factors of the thermal performance and their influencing regularity are discussed.The experimental results show that the system has good stability and reliability,and the ratio of the radiation heat-transfer rate to the gross heat-transfer rate is greater than 50%.When the floor structure and the cable distance are fixed,the gross heat-transfer rate of the upper floor surface has a maximum value at an optimal cable distance.Under the experimental conditions in this paper,the optimal cable distance is 50 mm.

Microstructures and properties of porous TiAl-based intermetallics prepared by freeze-casting

Preparation of porous Ti Al-based intermetallics with aligned and elongated pores by freeze-casting was investigated. Engineering Ti-43 Al-9V-1Y powder(D50=50 μm), carboxymethyl cellulose, and guar gum were used to prepare the aqueous-based slurries for freeze-casting. Results showed that the porous Ti Al was obtained by using a freezing temperature of -5 ℃ and the pore structure was tailored by varying the particle content of slurry. The total porosity reduced from 81% to 62% and the aligned pore width dropped from approximately 500 to around 270 μm, with increasing the particle content from 10 to 30 vol.%. Furthermore, the compressive strength along the aligned pores increased from 16 to 120 MPa with the reduction of porosity. The effective thermal conductivities of porous Ti Al were lower than 1.81 W/(m·K) and showed anisotropic property with respect to the pore orientation.

Eco-friendly synthesis of high silica zeolite Y with choline as green and innocent structure-directing agent

Zeolite synthesis in contemporary chemical industries is predominantly conducted using organic structure‐directing agents(OSDAs),which are chronically hazardous to humans and the environment.It is a growing trend to develop an eco‐friendly and nuisanceless OSDA for zeolite synthesis.Herein,choline is employed as a non‐toxic and green OSDA to synthesize high silica Y zeolite with SiO2/Al2O3 ratios of 6.5–6.8.The prepared Y zeolite samples exhibited outstanding(hydro)thermal stability at ultrahigh temperature owing to the higher SiO2/Al2O3 ratio.The XRF,SEM,29Si‐NMR and 13Na+results suggested that choline plays a structure‐directing role in the synthesis of Y zeolite,while the feed molar fraction of Na+is a crucial determinant for the framework SiO2/Al2O3 ratio and the crystal morphology.

Microstructure and thermal conductivity of carbon/carbon composites made with different kinds of carbon fibers

The microstructure and surface state of three kinds of polyacrylonitrile-based carbon fibers （T700, T300 and M40） before and after high temperature treatment were investigated. Also, the pyrocarbon and thermal conductivity of carbon/carbon composites with different carbon fibers as preform were studied. The results show that M40 carbon fiber has the largest crystallite size and the least d002, T300 follows, and TT00 the third. With the increase of heat treatment temperature, the surface state and crystal size of carbon fibers change correspondingly. M40 carbon fiber exhibits the best graphitization property, followed by T300 and then T700. The different microstructure and surface state of different carbon fibers lead to the different microstructures of pyrocarbon and then result in the different thermal conductivities of carbon/carbon composites. The carbon/carbon composite with M40 as preform has the best microstructure in pyrocarbon and the highest thermal conductivity.

Constructal entransy dissipation rate minimization for leaf-like fins

Based on constructal theory,the constructs of the leaf-like fins are optimized by taking minimum entransy dissipation rate(for the fixed total thermal current,i.e.,the equivalent thermal resistance) as optimization objective.The optimal constructs of the leaf-like fins with minimum dimensionless equivalent thermal resistance are obtained.The results show that there exists an optimal elemental leaf-like fin number,which leads to an optimal global heat conduction performance of the first order leaf-like fin.The Biot number has little effects on the optimal elemental fin number,optimal ratios of length and width of the elemental and first order leaf-like fins;with the increase of the thermal conductivity ratio of the vein and blade,the optimal elemental fin number and optimal ratio of the length and width of the elemental leaf-like fin increase,and the optimal shape of the first order leaf-like fin becomes tubbier.The optimal construct based on entransy dissipation rate minimization is obviously different from that based on maximum temperature difference minimization.The dimensionless equivalent thermal resistance based on entransy dissipation rate minimization is reduced by 11.54% compared to that based on maximum temperature difference minimization,and the global heat conduction performance of the leaf-like fin is effectively improved.For the same volumes of the elemental and first order leaf-like fins,the minimum dimensionless equivalent thermal resistance of the first order of the leaf-like fin is reduced by 30.10% compared to that of the elemental leaf-like fin,and the global heat conduction performance of the first order leaf-like fin is obviously better than that of the elemental leaf-like fin.Essentially,this is because the temperature gradient field of the first order leaf-like fin based on entransy dissipation rate minimization is more homogenous than that of the elemental leaf-like fin.The dimensionless equivalent thermal resistance defined based on entransy dissipation rate reflects the average heat transfer performance of the leaf-like fin,and can provide some guidelines for the thermal design of the fins from the viewpoint of heat transfer optimization.

Ballistic thermal transport in a cylindrical quantum structure modulated with double quantum dots

Ballistic thermal transport properties in a cylindrical quantum structure modulated with double quantum dots(DQDs) are investigated.Results show that the transmission coefficients exhibit the irregular oscillation.Some resonant transmission peaks and stop-frequency gaps can be observed,and the number and positions of these peaks and gaps are sensitive to the sizes of DQDs.With increasing the temperature,the thermal conductance undergoes a transition from the decrease to increase,and can be efficiently tuned by modulating the radius,length of DQDs as well as the interval between DQDs.In addition,at low temperatures,the enhancement of the thermal conductance can be also observed in this case.Some similarities and differences between the cylindrical and rectangular structures are identified.

Electrical and thermal transport properties of CdO ceramics

High temperature electrical and thermal transport properties,that is,electrical conductivity,Seebeck coefficient and thermal conductivity,of CdO ceramics have been investigated.Because of the good electrical properties and low thermal conductivity,the dimensionless figure-of-merit ZT of the CdO ceramics reaches 0.34 at 1023 K.This value is comparable to the best reported ZT for the n-type oxide ceramic thermoelectric materials and remains as potential to be further improved by porosity controlling or nanostructuring.