Experimental investigation of local heat transfer in the packing with triangular channels

Results of experimental studies on temperature distribution over the surface of a complex-shape heat exchanger like “Frenkel packing” are presented. Measurements were carried out with the airflow between two corrugated sheets with triangular crimps directed at 90° relative to each other. Microthermocouples glued on the outer surface of the heater were used for measurements. The effect of contact points, Reynolds number, and gap between corrugated sheets on temperature distribution over the heat exchanger surface is analysed under the turbulent mode of airflow. The main attention is paid to temperature distribution over the heating surface in an elementary cell. According to measurements performed, there is insignificant effect of contact points and gap on the type of temperature distribution over the perimeter of heated cell.     

Ultrahigh heat transfer enhancement using nano-porous layer

Heat transfer enhancement is one of the key issues of saving energies and compact designs for mechanical and chemical devices and plants. We discover an ultrahigh convective heat transfer performance compared to the well-known heat transfer correlations caused by a nano-particle porous surface: the maximum increase of heat transfer coefficient was around 180%. This nano-particle porous layer can be formed on the substrate surface by an etching with some acids or alkalis including around 100 nm size nano-particles made from copper oxide, carbon nano-tube and aluminium oxide. Moreover, we have done some experiments using a co-current flow heat exchanger consisting of hot and cold water-channels and obtained an ultrahigh heat transfer performance: over 200% increase compared to the conventional correlation. On the other hand, in order to theoretically investigate effects of nano-particle porous layer structures on the surface energy transfer, the energy transfer from fluid to the heat transfer surface is calculated by a classical molecular dynamics method. Energy transfer to the surface from the fluid strongly depends on the surface structures in nanometre scale that affect the static structure and dynamic behaviours of fluid molecules in the vicinity of the surface.     

Laser induced forward transfer of metal oxides using femtosecond double pulses

Laser induced forward transfer (LIFT) of ZnO and TiO₂ thin films onto Si has been realized with femtosecond double pulses of equal intensity, as a basic form of pulse shaping. In an effort to investigate the possibility to control the spatial distribution of the transfer via pulse separation, we used separation times varying from 0 to 10 ps. We have found that the size of the transferred spots increases by as much as 50% for pulse separation up to 500 fs, while for longer pulse separations the transferred spots acquire a constant area.     

Study of heat transfer characteristics in PCFG fabrication technology using heat method

The fiber gratings fabrication technology with the heating method in a photonic crystal fiber (PCF) based on structural change is examined. The principle of photonic crystal fiber gratings (PCFGs) is analyzed in theory. The heat transfer theory and finite element method are used to examine the thermal field distribution in the fiber and the influence of the air hole structure in the cladding, and the parameters of the laser beam in the process of grating fabrication are discussed. The results show that gratings can be formed by the periodic collapse of air holes in the cladding of PCFs. Under double-point heating condition, the energy is uniformly distributed in the radial direction and is approximate to Gaussian distribution in the axial direction. With the same size of the luminous spot, as the layers and radius of the air holes increase, the laser power needed to make the air holes collapse decreases. With the same laser power, as the luminous spot radius increases, the needed heating time increases. Moreover, the relationship between the laser power needed and the air filling rate is obtained as the number of layers of the air holes changes from 1 to 7. This kind of PCFG can overcome the long-term thermal instability of conventional gratings in substance and thus has great potential applications in the related field of optical fiber sensors.     

Improved transfer of quantum information using a local memory

We demonstrate that the quantum communication between two parties can be significantly improved if the receiver is allowed to store the received signals in a quantum memory before decoding them. In the limit of an infinite memory, the transfer is perfect. We prove that this scheme allows the transfer of arbitrary multipartite states along Heisenberg chains of spin-1/2 particles with random coupling strengths.     

Coherent control of Forster energy transfer in nanoparticle molecules: energy nanogates and plasmonic heat pulses

We study how Forster energy transfer from a semiconductor quantum dot to a metallic nanoparticle can be gated using quantum coherence in quantum dots. We show this allows us to use a laser field to open the flow of the energy transfer for a given period of time (on-state) before it is switched off to about zero. Utilizing such an energy gating process it is shown that quantum-dot-metallic-nanoparticle systems (meta-molecules) can act as functional nanoheaters capable of generating heat pulses with temporal widths determined by their environmental and physical parameters. We discuss the physics behind the energy nanogates using molecular states of such meta-molecules and the resonance fluorescence of the quantum dots.     

Chemical exchange saturation transfer MRI using intermolecular double-quantum coherences with multiple refocusing pulses

Chemical exchange saturation transfer (CEST) provides a new type of image contrast in MRI. Due to the intrinsically low CEST effect, new and improved experimental techniques are required to achieve reliable and quantitative CEST images. In the present work, we proposed a novel and more sensitive CEST acquisition approach, based on the intermolecular double-quantum coherence with a module of multiple refocusing pulses (iDQC-MRP). Experiments were performed on creatine and egg white phantoms using a Varian 7 T animal MRI scanner. The iDQC-MRP CEST technique showed a substantial enhancement in CEST and nuclear Overhauser enhancement (NOE) signal intensities, compared to the standard single-quantum coherence approach. In addition, the iDQC-MRP approach increased the signal-to-noise ratio of acquired saturation images, compared to the conventional iDQC approach. The new iDQC-MRP CEST sequence provides a promising way for exploiting in vivo CEST and NOE imaging applications.     

评估固体激光器对流换热系数的方法

 为提高固体激光器的热管理效果,提出了一种使用316L不锈钢片代替激光晶体评估固体激光器表面对流换热系数的方法。在低冷却水温的条件下,使用快响应热电偶对替代片和出口水流的动态温度进行测量,应用有限元方法计算不同对流换热系数下替代片的动态温度,讨论了对流换热系数对重频大能量激光器热效应的影响。通过寻找测量值和计算值的最小方差,得到该冷却结构下的激光器表面对流换热系数为3 500 W·m^-2·K^-1。     

Geometrical factors in the dispersion of ballistic heat pulses

The dispersion of ballistic heat pulses in dielectric crystals can partly be ascribed to geometrical factors that include the sample thickness, the contact areas of the source and detector and their relative positioning, and the acoustic anisotropy of the transmission medium. These factors are involved in determining the spread in the path lengths and group velocities of the phonons that are transmitted. In this paper we derive a simple expression, valid when the source and detector dimensions are small compared with the thickness of the slab-shaped sample, which relates the temporal width of a ballistic heat pulse to the average in-plane component of the slowness vector s of the transmitted phonons. A comparison is made between the predictions of this expression, Monte Carlo simulations and published experimental ballistic heat pulse data on paratellurite.     

Propagation of heat pulses in single-crystalline sapphire

We have studied heat-pulse propagation in single-crystalline sapphire using granular aluminum films for the heater and for the superconducting bolometer. The specimen surface carrying the bolometer was in direct contact with the liquid helium bath kept in the temperature range between 2.04 and 2.08 K. By varying the power during the heat pulse in the range between l mW and 2.8 W, we have observed the transition from purely ballistic pulse propagation at low power to diffusive propagation at higher power of the heat pulse. In the diffusive regime the phonon mean-free pathl has been experimentally determined and, using the Stefan-Boltzmann law, the variation ofl with the dominant phonon frequency has been found.     

Method for measurement the shape asymmetry of picosecond light pulses

Third order intensity correlation functions can be measured by experiments involving nonlinear optics. For a light pulse showing an asymmetric shape these correlation functions become asymmetric, too. Second and third order correlation functions are discussed for an exponential pulse shape.     

Focusing of heat pulses along nonequilibrium nanowires

As a consequence of the temperature dependence of the speed of heat pulses, rectangular heat pulses will shrink (or extend) spatially, and will increase (or decrease) their temperature when propagating along a temperature gradient. Here, we consider heat pulse propagation along silicon nanowires, because of their interest in nanotechnology. The relative rates of variation per meter may be very high, and variations along relatively short lengths could be experimentally observable.     

Studies on widely tunable ultra-short laser pulses using energy transfer distributed feedback dye laser

This paper presents both theoretical and experimental study of the characteristics of Nd: YAG laser pumped energy transfer distributed feedback dye laser (ETDFDL). Using theoretical model proposed, the behavior of ETDFDL such as the characteristics of donor DFDL, the acceptor DFDL, the dependence of their pulse width and output power on donor-acceptor concentrations and pump power are studied for dye mixture Rhodamine 6G and Cresyl Violet in detail. Experimentally using prism-dye cell configuration, the ETDFDL output is obtained and the output energy of DFDL is measured at the emission peaks of donor and acceptor dyes for different pump powers and donor-acceptor concentrations. In addition, the DFDL linewidth measurement has been carried out at the lasing wavelengths of the donor and acceptor dyes using Fabry-Perot etalon and the tunability of DFDL is measured to be in the wavelength range of 545-.     

The conjugate problem of convective-conductive heat transfer for heat pipelines

The conjugate problem on heat transfer in a system of subsurface channel one-duct heat pipeline-environment is solved by the finite element method. Thermal regimes of the heat pipeline are analyzed, depending on the temperature at the internal boundary of the system. Values of the main characteristics of the system are found and adequacy of the proposed model is validated.     

高效换热器内冷凝换热热阻的实验研究

在水平强化管内R134a的冷凝换热实验中,对试验工况中管内流型进行计算预测,使用关联式对管内传热系数进行预测,以校核实验数据的可靠性,以质量流量、冷凝温度为变量,以总传热系数K、冷冻水传热系数h_w、制冷剂传热系数h_r为衡量指标,对强化管换热热阻进行研究。实验结果表明:总传热系数K、制冷剂传热系数h_r均随质量流量的增加而增大,且对应冷凝温度越低其值越大,而冷冻水传热系数h_w随质量流量的增加而稍有降低,冷凝温度对其值影响并不大。此外,随着质量流量的增加,冷冻水热阻占总热阻比值逐渐增加,但制冷剂热阻总是小于冷冻水热阻,且冷凝温度越低,两者的差值越大。换热管内外热阻比重IOR随着质量流量的增加而降低,随着冷凝温度的升高而增大。     

Revealing local field structure of focused ultrashort pulses

We utilize the interaction of tightly focused ultrashort pulses with transparent media to imprint their local polarization in the focal region. In particular, we demonstrate that this technique allows for a subwavelength resolution diagnostic of complex polarization states including the presence of the longitudinal component of the electric field. Moreover, we demonstrate the first ever material ablation with the longitudinal electric field of femtosecond pulses.