Basic Heat Transfer Characteristics Research Articles

Experimental investigation on subcooled boiling heat transfer of emulsified kerosene

To investigate heat transfer characteristics of emulsified kerosene in a mini tube, experiments were conducted at the pressure of 3.0–5.0 MPa, mass flow rate of 0.8–2.4 g·s−1, and water mass fraction of 10%–50%. Water in oil type emulsion was prepared using the surfactant of span-80. The basic heat transfer characteristics of emulsified kerosene covering a wide temperature range were revealed. More attention was focused on subcooled boiling of emulsified kerosene. Depending mainly on low-boiling temperature dispersed phase of water, the heat transfer of emulsified kerosene could be divided into 6 regions: single liquid-phase convection, subcooled boiling, saturated nucleate boiling, film boiling, steam-kerosene convection, and single gas-phase convection. Heat transfer deterioration characterized by a significant soar in wall temperature could be observed in the film boiling region. In the fully developed subcooled boiling region, higher heat transfer coefficient could be achieved at lower pressure. With an increase in mass flow rate, subcooled boiling was postponed to higher heat flux. At the same heat flux, the fluid temperature was negatively correlated with the water mass fraction. However, the effect of water mass fraction on inner wall temperature could be ignored. The subcooled boiling heat transfer was enhanced at lower water mass fraction. Most of the existing enhancement-factor type correlations underestimated the subcooled boiling heat transfer of emulsified kerosene. According to the experimental data, a new enhancement-factor type correlation was proposed for subcooled boiling heat transfer of emulsified kerosene. The mean absolute deviation of the new correlation was 9.9%.

Development of magnetically-driven cooling device with concentric pipe structure

Temperature-sensitive magnetic fluid is a smart material for energy carrier. The most interesting aspect of temperature-sensitive magnetic fluid is that the thermal flow behavior is actively controlled by means of magnetic field. Based on the effect of the temperature-dependent magnetization, temperature-sensitive magnetic fluid can be utilized as an energy conversion system, which can automatically transfer the thermal energy. The advantage in the engineering application can be derived from the fact that there would be entirely no external energy consumption, with which large amount heat can be transported for a long distance without any external power consumption. Taking into account of the advantage, a magnetically-driven cooling device is newly designed for recovering of low- to high-temperature waste heat in the present study. The basic performance of the cooling device with concentric pipe structure is investigated experimentally and data gained in the device is examined in detail in view of magneto-hydrodynamics. In the present study, electromagnet is used as an external magnetic field for the purpose of investigating basic heat transfer characteristics of the present experimental device, so that the magnetic field can be continuously altered. However, it can be easily replaced to a permanent for the practical device without additional electrical energy. The results show that the binary temperature-sensitive magnetic fluid can be circulated freely with a high flow rate of 2.0 × 10−3 m3/min by imposing the magnetic field of 55.8 kA/m. It is found that the newly designed device can transfer thermal energy more than 250 W with overall system efficiency of 11.0% at air temperature of 623 K.

Thermal performance analysis and optimization of a cascaded packed bed cool thermal energy storage unit using multiple phase change materials

This paper proposes a cascaded packed bed cool thermal energy storage (CTES) unit using multiple phase change materials (PCM). In terms of the solidification processes, the basic heat transfer characteristics were analyzed based on an experimentally validated simulation model. Thermal performances, including cold charging rate, the quantity of cold and exergy charged, as well as the exergy efficiency of the cascaded CTES unit were compared with those of a single-stage CTES unit. In order to further achieve better thermal performance, the materials, stages and thickness of each stage were optimized. According to the results, the 24-stage CTES unit with a phase change temperature difference between the highest and lowest stage (denoted as Td) of 6 °C has the best thermal performance, because it shows a 15.1% reduction in charging time compared with a single-stage unit, while its quality and quantity of cold charged remains almost equivalent to the single-stage unit. Moreover, 3–5 stages are recommended for an evenly distributed cascaded CTES as their thermal performances are quite close to the optimal 24-stage unit.

Flow boiling heat transfer of binary mixed magnetic fluid

A new type of binary magnetic fluid, which is a mixture of base magnetic fluid with an organic secondary fluid, whose saturation temperature is lower than that of the base magnetic fluid, was introduced. Basic heat-transfer characteristics of the binary magnetic fluid were examined for boiling two-phase flow in a partly heated pipe. It was indicated that continuous boiling was achieved without any deterioration in magnetic properties of the fluid and the heat-transfer rate of the flow was improved remarkably by an applied magnetic field at higher flow rate.