Flow patterns for water-butanol, water-toluene and water-hexane two-phase flows were visualized in the cross-shaped junctions of three square glass microchannels with hydraulic diameters of 200 μm, 400 μm and 600 μm. Typical flow pattern maps for water-butanol two-phase flow were developed based on Capillary number of the continuous phase and Weber number of the dispersed phase, and compared with a previous flow pattern map in the literature. Three main flow pattern groups were observed, including the tubing/threading regime group, the dripping regime and the jetting regime. The geometry confinement and fluid properties affected the viscous shear and interfacial tension forces and therefore their influence on flow pattern transitions was clarified. Besides, in the dripping regime, the dimensionless slug length can be scaled as a function of the flow rate ratio and the Capillary number of the continuous phase.

An experimental investigation of an oscillating heat pipe (OHP) with an inner surface coated with a copper oxide (CuO) layer was conducted. The OHP has six turns and three sections: evaporator, condenser and adiabatic section with the lengths of 40 mm, 64 mm and 51 mm, respectively. The cleaned copper tubing was chemically treated with a chemical solution and heated in a furnace. A layer of CuO was formed in the inner surface of the OHP. A working fluid (water in this study) at filling ratios ranging from 40% to 70% was studied. The experimental results show that the CuO layer can enhance the heat transfer performance of the OHP. The investigation results in a new way to enhance the heat transfer performance of an OHP.

This paper presents an experimental investigation on the co-current downward condensation of R134a inside a tube-in-tube heat exchanger. The test section is a 0.5 m long double tube with refrigerant flowing in the inner tube and cooling water flowing in the annulus. The inner tube is constructed from smooth copper tubing of 9.52 mm outer diameter and 8.1 mm inner diameter. The condensing temperatures are between 40 and 50°C, heat fluxes are between 9.78 and 50.69 kW m −2 . The temperature difference between the saturation temperature of refrigerant and inlet wall varies between 1.66–8.94°C. Condensation experiments are done at mass fluxes varying between 340 and 456 kg m −2 s −1 while the average qualities are between 0.76–0.96. The quality of the refrigerant in the test section is calculated considering the temperature and pressure measured from the test section. The pressure drop across the test section is directly measured by a differential pressure transducer. The average experimental heat transfer coefficient of the refrigerant is calculated by applying an energy balance based on the energy transferred from the test section. Experimental data of annular flow are examined such as the alteration of condensation heat transfer coefficient with the vapor average quality and temperature difference respectively according to different mass fluxes and condensing temperatures. The relation between the heat flux and temperature difference, besides this, the relation between the condensation heat transfer coefficient and condensing pressure are shown comparatively and the effects of mass flux and condensation temperature on the pressure drop are also discussed. The efficiency of the condenser is considered comparing with various experimental data according to tested condensing temperatures and mass fluxes of refrigerant. Some well known correlations and models of heat transfer coefficient were compared to show that annular flow models were independent of tube orientation provided that annular flow regime exists along the tube length and capable of predicting condensation heat transfer coefficient in the test tube.

NASA plans human exploration near the South Pole of the Moon, and other locations where the environment is extremely cold. This paper reports on the heat transfer performance of a loop heat pipe exposed to extreme cold under the simulated reduced gravitational environment of the Moon. A common method of spacecraft thermal control is to use a loop heat pipe with ammonia working fluid. Typically, a small amount of heat is provided either by electrical heaters or by environmental design, such that the loop heat pipe condenser temperature never drops below the freezing point of ammonia. The concern is that a liquid-filled, frozen condenser would not re-start, or that a thawing condenser would damage the tubing due to the expansion of ammonia upon thawing. This paper reports the results of an experimental investigation of a novel approach to avoid these problems. The loop heat pipe compensation chamber is conditioned such that all the ammonia liquid is removed from the condenser and the loop heat pipe is non-operating. The condenser temperature is then reduced to below that of the ammonia freezing point. The loop heat pipe is then successfully re-started.

In this paper are presented the results of a thermal analysis on a high throughput droplet based, nucleic acid amplifier. Initial data of successful amplification of DNA is also presented. The advent of microfluidics offers many opportunities to integrate all functional steps of DNA analysis onto a single platform allowing for reduced analysis times, reduced sample volume requirements and higher throughput. There are many technical challenges facing this goal, thermal control being pivotal. This paper involves fundamental theoretical and experimental characterization of the core element of such a platform; a polymerise chain reaction (PCR) thermal cycler. The PCR process can be either a two-step or three-step temperature cycling procedure, depending on the chemistry involved. Presented in this paper is a thermal analysis of four, two-step thermocyclers arrayed in parallel. Two step PCR requires samples to be cycled through temperature ranges of 92–95°C and 60°C, the preciseness of these temperatures again depending on the chemistry involved. For optimum efficiency, fast heating and cooling between steps, and uniformity within each step is crucial. Our thermocycler design comprises a flow conduit in a serpentine pattern, machined in two segmented aluminium blocks, in which the conduit extends through the denaturation (92–95°C) and the annealing and extension (60°C) zones. Circular tubing, in which the samples are passed through to ensure biocompatibility for the reaction, is embedded in the machined channel which results in high heat transfer from the block to the sample. The device is then positioned in the vertical plane and an array of thermocyclers are formed by combining multiple planar systems. Thermofoil heaters are attached to the underside of the upper blocks creating the denaturation temperature, and by adjusting the air gap, passive thermal control is used to create the temperature required for the annealing and extension zone, thereby reducing the number of heaters and thus the power required. Theoretical thermal analysis of the device is performed, in conjunction with CFD simulations, experimental testing, and thermal imaging. Finally successful PCR of the gene ABL1 was performed.

Improved miniaturization and a trend towards increasingly dense and compact architectures have led to unmanageably high heat fluxes in electronic components. In order to keep temperatures at operational levels more advanced cooling solutions are being required that go beyond the solid heat sink and forced convection. Pulsating heat pipes made out of multi port extrusion tubing are a proposed solution. Typically, gas-liquid slug flow occurs in the serpentine channel imbedded in the pulsating heat pipe. Vapour is produced in the heated section and condensed in the cooled section located at opposite ends of the heat pipe. In this work, experiments were conducted on four Multi-Port Extruded (MPE) aluminum tubing heat pipes with different internal structures: rectangular channel looped, rectangular channel unlooped, triangular channel looped, and triangular channel unlooped. The effect of changing the working fluid (ethanol or de-ionized water), fill ratio, and orientation were measured and compared for the different heat pipes. It was found that most of the heat pipes performed better with ethanol than de-ionized water. Only the looped rectangular channel heat pipe performed satisfactorily with de-ionized water, which is attributed both to the larger channel size and the looped architecture. The unlooped heat pipes performed best at the lowest fill ratios (10%) while the looped heat pipes showed their best performances between 30 and 50% with marked decrease at the lower and higher fill ratios. Both looped heat pipes performed poorly in horizontal orientation as compared to vertical, however, the unlooped heat pipes performed quite well in both orientations. This may be more the effect of the fill ratio on horizontal performance as literature suggests that horizontal orientation requires a lower fill ratio to perform satisfactorily.

A hypothesis is evaluated for a miniature shell and tube heat exchanger that the distribution of flow through the tubes is non-uniform when steam condenses inside the tubes. It is believed that due to the small tubing size, surface tension effects, generally negligible in larger exchangers, will have a noticeable effect on distribution of condensate through the tube bundle. This hypothesis was tested using a custom-built miniature shell and tube heat exchanger. The unit consisted of nineteen 0.094” OD tubes (0.010” wall thickness) in a 3/4” OD shell. The effect of both angle of inclination (0°, 45° and 90°) and steam flow regime (loosely defined as low, medium and high) on fluid distribution were investigated. Initial results proved that the flow distribution during condensation on the tube side was non-uniform; within a given trial, each tube produced a significantly different amount of condensate. Furthermore, while at a horizontal angle this non-uniformity was observed to be sporadic with respect to tube location. A tube that produced little condensate in one trial might produce a substantial amount of condensate in a subsequent trial. With increasing angle of inclination, the amount of fluid condensed became more consistent with respect to tube location within the bundle. The experiments were meant to be a preliminary investigation to either prove or disprove the hypothesis. As experimental results did not disprove the hypothesis, future study is recommended. More research into the effects of angle and flow rate on distribution should be conducted. Additionally, the effect of tube size should also be studied. If surface tension is the true explanation for the observed phenomena, it is expected that as the tube size increases, the amount of mal-distribution will be minimized.

A small-scale loop heat pipe (LHP) with polypropylene (PP) wick was fabricated and tested for its thermal performance. The container and tubing of the system were made of stainless steel and several working fluids were used including methanol, ethanol, acetone, and ammonia. The heater and the evaporator were sized so that the system can be applied to a local thermal control including electronics cooling. The heating area was 35 mm × 35 mm and there were nine axial grooves in the flat evaporator (40 by 50 mm) to provide a vapor passage. The pore size of the polypropylene wick inside the evaporator was varied from 0.5 μm to 25 μm. The size of condenser was 40 mm (W) × 50 mm (L) in which ten coolant paths were provided. The inner diameters of liquid and vapor transport lines were 2.0 mm and 4.0 mm, respectively and the length of the two lines was 0.5 m each. The start-up transient as well as steady-state operation was investigated with maximum system operating temperature of 90°C, which was imposed to protect the PP from permanent deformation. The minimum thermal load of 10 W (0.8 W/cm 2 ) and maximum thermal load of 80 W (6.5 W/cm 2 ) were achieved using methanol as working fluid with the condenser temperature of 20°C at horizontal position. For a LHP with ammonia as working fluid, the minimum thermal load of 1 W and maximum thermal load of 87 W (7.1 W/cm 2 ) were achieved for condenser temperature of 0°C at horizontal position. The minimum system thermal resistance was 0.65 K/W.

The pressure drop and boiling heat transfer of steam water two-phase flow were studied in a small horizontal helically coiled tube once-through steam generator of 9-mm inside diameter with 292-mm coil diameter and 30-mm pitch. Experiments were performed at a range of qualities up to 0.95, a system pressure range of 0.5∼3.5MPa, a mass flux range of 236∼943kg/m 2 s and a heat flux range of 0∼900kW/m 2 . Based on the experimental results, a new two-phase frictional pressure drop correlation was developed on the basis of Chisholm’s B -coefficient method. In the present experimental range, boiling heat transfer was found to be dependent not only on mass flux but also on heat flux. This result implies that both the nucleation mechanism and the convection mechanism have the same importance to forced convective boiling heat transfer in small horizontal helically coiled tube over the full range of qualities (pre-critical heat flux qualities of 0.1∼0.9) which is contrary to situations in larger helically coiled tube where the convection mechanism dominates at qualities typically > 0.1. Traditional single parameter Lockhart-Martinelli type correlations failed to satisfactorily predict present experimental data and in this paper a new flow boiling heat transfer correlation was put forward to better predict the experimental data of the present study.