Aim:This study was designed to evaluate the influence of different time intervals of a vital home bleaching procedure on the fracture toughness of bovine enamel.Materials and Methods:Labial aspects of 48 bovine incisors were prepared and stored in artificial saliva. The samples were divided into four groups (n = 12) according to the different time intervals of the bleaching procedure: 0, 2, 4, and 6 weeks. The experimental groups were subjected to the bleaching agent; 15% Opalescence PF according to manufacturer's instructions. The fracture toughness (FT) of enamel was assessed using Vickers hardness indentation. Then, FTs were compared statistically by one-way ANOVA (α = 0.05).Results and Discussion:Different tested time intervals did not significantly influence the fracture toughness of bovine enamel following treatment with 15% carbamide peroxide (P = 0.05)Conclusion:15% Opalescence PF after 6 weeks did not reduce the fracture toughness of human enamel compared with the baseline, 2 and 4-week values.

The solar air heater is a widely used thermal application of renewable energy. Various thermal storage materials, either sensible or latent, could be adopted for solar air heater, so that the thermal energy can be saved at sunny hours for use after the sunset. Considering its latent heat of fusion, paraffin wax, a phase change materials (PCM) was selected to store latent thermal energy, applied on a forced convection solar air heater (airflow: 4.47 kg/s), and compared with a sensible thermal storage material under the meteorological conditions of Baghdad city. Three tests were conducted on February 24, 25 and 27, 2019, respectively. In the first test, no thermal storage material was used; in the second test, pure cement was adopted as the sensible thermal storage material (specific heat: 1.55 kJ/kg· K; thermal conductivity: 0.29 W/m· K); in the third test, paraffin wax was selected as the latent thermal storage material (solid phase: specific heat: 2.0kJ/kg· K; thermal conductivity: 0.24 W/m· K; liquid phase: specific heat: 2.15kJ/kg· K; thermal conductivity: 0.22 W/m· K). The results show that the latent thermal storage material of paraffin wax achieved better thermal performance than the sensible thermal storage material of pure cement; the solar air heater with paraffin wax had a 50% higher thermal storage efficiency and realized twice the heating period after 6:30pm than that with pure cement.

In this study, two approaches to investigating the process cooling of a heated sphere were performed using air as well as air/water mist two-phase flow. Steady-state and unsteady heat transfer analysis were compared in the terms of the average surface temperature and heat transfer rate between the sphere surfaces and the cooling fluid. When the Bi<0.1, the temperature variation inside sphere can be neglected and the wildly known lumped capacitance model can be applied to estimate the heat transfer coefficient by measuring the sphere surface temperature. The effect of the different factors such as the inlet Re numbers, surface temperature and water mist rate on heat transfer characteristics are examined. The experimental results showed that the presence of water mist leads to a significant increase in heat transfer rate over the use of air coolant alone. Also, the unsteady thermal behaviors of the water mist impingement on the heated surface and dynamicstate of cooling process changing over the sphere surface were analyzed experimentally based on the unsteady surface temperature measurements. The experimental results of the unsteady heat transfer were compared to the results obtained from steady-state estimation under the corresponding surface temperature of the sphere. Moreover, the new proposed empirical correlation for the Nusselt number based on the present experimental data are given for practical uses. Results reasonably agree well within ±3.8%.

No abstract

Demands to increase the safety and reliability of the modern nuclear reactors are constantly being made. Typical demands include the increases the thermal performance of the passive heat removal system PHRS by removing larger amounts of energy and applied a technique that leads to reduce size and weight of the PHRS unit. This article presents an experimental investigation on the natural convection based on alternative cooling approach using water mist. The results obtained from the related experimental work indicated that the heat transfer rate was enhanced over that for the airflow as a result of water mist evaporation on the surface of the tubes. The Nu number increases by about 148%, 144%, and 128% respectively for all tube rows compared with airflow. The experimental results of the thermal performance factor values obtained by suspended water mist were correlated as functions of Rayleigh number (Ra) and Weber number (We). Subsequently, the predicted thermal performance factor from the correlation was plotted to compare with the experimental data. It was found that the thermal performance factor was within ±11%.