Development of an Adjustable Spiral-shaped Evaporator

The cooling of high heat flux is becoming increasingly important in technical applications. This is on the one hand due to the fact that the available installation space is becoming smaller and smaller due to progressive miniaturization, on the other hand to the ever-increasing performance, which has to be taken away in technically demanding processes such as Inconel machining. In order to meet this challenge, a new type of evaporator, the swirl evaporator, was developed. The swirl evaporator is a screw-shaped cylindrical evaporator with an internal diameter between 1 – 3 mm, which is inserted as a blind hole in components with high heat generation. The refrigerant is fed into the blind hole via a concentrically oriented capillary, deflected by 180° in the drilling base and flows out of the evaporator again in a helical way (twist flow) against the inflow direction. The evaporator\u27s design allows a compact size to be achieved, making it suitable for a wide range of technical applications. To enable a design for industrial needs, a 1-D simulation of the process had been conducted. The simulations showed ideal results for a hydraulic diameter of 2.05 mm and an evaporator length of 15 – 20 mm. According to the simulations an improvement of the energy efficiency of up to 19 % is possible when the mass flow is kept constant for R32. Based on the results of these simulations a design of the test-rig had been developed which allows different screw inserts to be tested with a variable length. Former experimental studies with R404A show that the average critical heat flux density of spot evaporators with a twist geometry increases by up to 33 % compared with spot evaporators without twist generation. The spot evaporators with swirl flow generation have a distinct, stable overheating zone with high heat flux (Humpfer, 2013)

Hydraulic Characterization of a Adjustable Spiral-shaped Evaporator

To ensure reliability in miniaturized devices or processes with increased heat fluxes, decreasing available cooling surfaces have to be met by novel cooling methods. In order to meet this challenge, a new type of evaporator, the swirl evaporator, was developed. The swirl evaporator is a screw-shaped cylindrical evaporator with an internal diameter between 1 – 3 mm, which is inserted as a blind hole in components with high heat generation. The refrigerant is fed into the blind hole via a capillary, deflected by 180° in the drilling base and flows out of the evaporator again in a helical way (twist flow) against the inflow direction. The refrigerant is pressed against the hot wall by centrifugal forces. This results in an increased critical heat flux. The evaporator\u27s design allows a compact size, making it suitable for a wide range of technical applications. To enable its design for industrial needs, a test stand has been developed. The hydraulic characterization was performed by varying the evaporator length. First experimental results show a linear relationship between swirl evaporator length and pressure loss

Elementary excitations of the symmetric spin-orbital model: The XY limit

The elementary excitations of the 1D, symmetric, spin-orbital model are investigated by studying two anisotropic versions of the model, the pure XY and the dimerized XXZ case, with analytical and numerical methods. While they preserve the symmetry between spin and orbital degrees of freedom, these models allow for a simple and transparent picture of the low--lying excitations: In the pure XY case, a phase separation takes place between two phases with free--fermion like, gapless excitations, while in the dimerized case, the low-energy effective Hamiltonian reduces to the 1D Ising model with gapped excitations. In both cases, all the elementary excitations involve simultaneous flips of the spin and orbital degrees of freedom, a clear indication of the breakdown of the traditional mean-field theory.Comment: Revtex, two figure

Thermodynamics of the one-dimensional SU(4) symmetric spin-orbital model

The ground state properties and the thermodynamics of the one-dimensional SU(4) symmetric spin system with orbital degeneracy are investigated using the quantum Monte Carlo loop algorithm. The spin-spin correlation functions exhibit a 4-site periodicity, and their low temperature behavior is controlled by two correlation lengths that diverge like the inverse temperature, while the entropy is linear in temperature and its slope is consistent with three gapless modes of velocity $\pi/2$. The physical implications of these results are discussed.Comment: 4 pages, 4 figures, RevTe

A-type Antiferromagnetic and C-type Orbital-Ordered State in LaMnO3_3 Using Cooperative Jahn-Teller Phonons

The effect of Jahn-Teller phonons on the magnetic and orbital structure of LaMnO$_3$ is investigated using a combination of relaxation and Monte Carlo techniques on three-dimensional clusters of MnO$_6$ octahedra. In the physically relevant region of parameter space for LaMnO$_3$, and after including small corrections due to tilting effects, the A-type antiferromagnetic and C-type orbital structures were stabilized, in agreement with experiments.Comment: 4 pages, Revtex, with 3 figures embedded in the text. Submitted to Phys. Rev.