A system that releases heat to the surroundings, an exothermic reaction, has a negative ΔH by convention, because **the enthalpy of the products is lower than the enthalpy of the reactants of the system**. The enthalpies of these reactions are less than zero, and are therefore exothermic reactions.

Convective heat transfer coefficient doesn't depend on

Explanation: It is denoted by h and is dependent on **space, time, geometry, orientation of solid surface**.

To answer the title of your question, yes it is not only customary, **it is necessary to express heat loss as a negative number**, but the question still has to be phrased correctly--using the term "lost" serves as a negation in this use of language, the same way the unary operator in -1 serves as negation for a number.

Convection is the bulk transfer of fluids. In this mode, it is because of mass transfer. **When there is an increase in velocity there will be an increase in mass transfer**. When the mass transfer increases there is an increase in the heat transfer coefficient.

If Q is less than zero or a negative number, this means that **the temperature has gone down**. The substance has lost heat, so the heat is negative. The final temperature is lower than the initial temperature, so the sign will be negative.

If ∆H is negative, this means that the reaction gives off heat from reactants to products. **This is favorable**.

**Positive heat transfer is when the temperature goes up with the length.** **Negative heat transfer is when the temperature gradient decreases**.

negative

According to the classical sign convention, **heat transfer to a system and work done by a system are positive**; heat transfer from a system and work done on a system are negative.

Assuming the heat transfer surface and temperature difference remain unchanged, **the greater the U value, the greater the heat transfer rate**. In other words, this means that for a certain heat exchanger and product, a higher U value could lead to shorter batch times and increased production/revenue.

So, if a reaction releases more energy than it absorbs, the reaction is exothermic and enthalpy will be negative. Think of this as an amount of heat leaving (or being subtracted from) the reaction. **If a reaction absorbs or uses more energy than it releases, the reaction is endothermic, and enthalpy will be positive**.

Typical convective heat transfer coefficients for some common fluid flow applications: Free Convection - air, gases and dry vapors : **0.5 - 1000 (W/(m ^{2}K))** Free Convection - water and liquids: 50 - 3000 (W/(m

A system that releases heat to the surroundings, an exothermic reaction, has a negative ΔH by convention, because **the enthalpy of the products is lower than the enthalpy of the reactants of the system**. The enthalpies of these reactions are less than zero, and are therefore exothermic reactions.

In case of constant wall temperature, using adiabatic wall temperature as reference temperature can result in negative heat transfer coefficient, which means **the heat flux has a different direction with the defined driving temperature difference**.

As the position x increases, the thickness of the thermal boundary layer increases, i.e. the temperature increases over a greater distance y. Thus, **the temperature gradient decreases in the x-direction and so does the local heat transfer coefficient**.

The larger the heat transfer coefficient, **the more heat transfer occurs**. In general, the larger the thermal conductivity for a fluid, the larger the heat transfer coefficient. Therefore, the heat transfer coefficient for a liquid is higher than that of a gas.

Enthalpy of a reaction is defined as the heat energy change ( Δ H ΔH ΔH ) that takes place when reactants go to products. **If heat is absorbed during the reaction, Δ H ΔH ΔH is positive; if heat is released, then Δ H ΔH ΔH is negative**.

As the mass flow may be calculated with **dm = dv x ρ (volume flow times density)** we will get the "qualitative" Heat transfer coefficient α_{m} . Using: T2, T_{1} the inlet and outlet temperature of the fluids and the volume flow. The heat loss is also taken as constant.

12-Jul-2022

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